There is a physical threshold, Jose Garces concedes, when he'll no longer be able to staff and execute another new restaurant at the lofty level his public has come to expect.

And he says JG Domestic, his sprawling new homage to artisan American ingredients in the modern lobby of the Cira Centre, has pretty much pushed the Iron Chef right up to the edge of that perilous precipice: "This was more than likely going to be my last full-service, go-for-it restaurant," he says.

I'm not buying the vow of ambition abstinence from one of our city's most voracious restaurant czars quite yet. But it's still good to hear the man has some sense of limits. Because you wouldn't know it from the setup here.

It took some bravado - and awesome tax incentives, plus office space! - to take over this rambling dining room on the ground floor of the skyscraper attached to 30th Street Station, where a chunk of the seats are tucked beneath an escalator, the monogram name sounds like a corporate patent firm, and the locale, sliced off from local civilization by a whorl of highway, train track, speeding taxis, and grouchy parking attendants, is about as approachable as a SEPTA third rail.

But then the homey Parker House rolls arrive, still glistening warm in their cast-iron pan beneath a dusting of sea salt with whipped Vermont butter and a herbaceous pat of apple gelée on the side. I take a bracing sip of a favorite new bourbon drink scented with orange and house-made coffee bitters, the Empire Builder, named for a Midwestern Amtrak passenger line.

And before long, a long train of evocative little plates is chugging across my table - hickory-smoked pecans tinged with maple and bacon; crunchy little scroll-shaped tubers called crosnes, dolloped with dusky clouds of smoked ricotta; a froth-topped cup of bisquey lobster "cappuccino," with tiny squash ravioli bobbing beneath the foam; whole animals braised, seared, sous-vided, and confited down into little piles of sublimely tender heirloom meat - only to leave my station completely clean.

Garces may well someday run out of gas to keep the concepts hot, fresh, and exciting. But with talented young chef de cuisine David Conn running the Cira Centre show, it hasn't come yet. And ultimately, having a meal at JG Domestic is to be reminded of the deep stores of untapped culinary horsepower that still drive this Garces Restaurant Machine - despite its occasional minor flaws.

The space inherited from Rae, for example, is not my favorite, with disjointed seating that rambles from the airy lobby back past the bar and communal table, along the deep booth alcoves beside the glassed-in kitchen, and into a warren of private rooms. Designer Jun Aizaki has done what he can to warm the vibe, with lots of exposed butcher-block wood for the picnic-table effect, rustic tchotchkes, plaid booths, and greenery to soften the contemporary edges just enough.

The service is friendly yet refined, with charming waiters like Thomas Dean and Nathan Anderson lending enthusiastic knowledge to the ever-changing food and drink menus.

It is the showcasing of Conn's skills, though, that really makes this destination most worth seeking. Conn, 28, a Garces lieutenant since the beginning, has channeled the Iron Chef's more exotic inspirations from Spain (Amada, Tinto), Mexico (Distrito), and Asia and Peru (Chifa). It's intriguing now to see what flavors from American soil move the two of them.

Gratefully, JG's mission is more simply about sourcing great U.S. ingredients - Marvesta prawns from Maryland, Jidori chicken, Lancaster kabocha squash, Keswick Creamery cheese - than an exercise in reinventing a patchwork of cliched regional specialties. The ingredients should only improve as Garces' new Bucks County farm comes on line in the coming year.

But already, this kitchen is flexing its muscle through vivid seasonal flavors drawn from across the land, and, as a result, more spontaneity than most any other Garces restaurant. The brick-oven flatbread delivered an earthy whiff of Oregon truffles and chanterelles scattered over creamy white asparagus béchamel piqued with Jasper Hill cheddar. Tiny croquettes made from Lancaster sugar pumpkins burst inside the mouth like crispy capsules of warm squash soup. Little mason jars of crudité showcased winter produce at its most precious moment of crunch - tiny florettes of gold and purple cauliflower, blush-tipped radishes, baby turnips and carrots - illuminated with California olive oil and a tarragon froth of Green Goddess.

Tender Texas boar chops, in season since October, bring an intense savor to Anson Mills grits kissed with just a peck of maple sweet. I've heard complaints about skimpy portions on that dish. Mine was ample, but there are other plates that could spark value gripes: The seared black cod over salsify puree was fantastic, but minuscule for $25.

Hefty portions have never been a Garces hallmark (though the tasty roasted Jidori chicken, at $28, could easily feed two). I've always found true value, though, in the focus of flavors and the artistry of the cooking. And with only a couple misfires on that end - sticky-sauced lamb spare ribs that were too fatty, a crab gratin that was both boring and ripe - JG's kitchen is no exception.

So many of these dishes were memorable, from the nibbles (fluffy Iowa popcorn tossed in micro-planed horseradish and Vermont cheddar; a quiver of house-made Berkshire pork salami sticks, the slim JG's that are the ultimate snack for train travel) to any number of more involved plates. The potted duck was a decadent parfait of juniper-scented duck rillettes layered with creamy foie gras mousse and orange gelée. The pepper-edged sheets of Strube Ranch Wagyu beef carpaccio melted away on the tongue with a gorgeously marbled rare beefiness that was chased by dabs of creamy goat cheese, sweet beet puree, and toothy pistachios.

A cooked slab of rib-eye from Kansas evoked the Southwest with a dried chile rub, addictive Rancho Gordo beans refried with bacon fat, and onion rings crisped in cornmeal with a hint of celery seed. Sublimely tender head-on Maryland prawns basked in a tomato broth perked with Meyer lemon.

The "whole animal" feature, though, is this menu's most intriguing option. Changing constantly with availability, it challenges the cooks to present multiple cuts and techniques on a single plate of the best possible meats - Elysian Fields Pennsylvania lamb (oh, that chop!), Griggstown Farms pheasant (the ultimate bird confit), and an Arkansas rabbit that was my favorite, from the brandy- and mustard-braised forequarters to the butter-soft "rack," radiating bones as delicate as toothpicks.

Follow that up with a maple souffle, or an elegant apple parfait, or those sugar-flocked beignet pillows drizzled in hot streams of Maker's Mark butterscotch. The only physical threshold really being tested at JG Domestic will be your ability to stop eating.

Next Sunday, Craig LaBan reviews Baby Blues BBQ in University City. Contact him at claban@phillynews.com.