The pulse of an incoming call waiting keeps breaking into our phone interview, so I offer to pause on hold. But Mark Tropea, the chef and owner of Sonata in Northern Liberties, doesn't flinch after a glance at caller ID.

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"It's a bill collector," he says. "They can wait for you."

Ah, that "wait" - for more customers, for enough momentum to turn a profit, for praise (or, failing that, just recognition that Sonata exists) - it's a potentially numbing state of off-the-radar limbo that Tropea has refined with admirable determination.

The 30-year-old Glen Mills native, a Restaurant School grad and longtime stalwart at the Desmond in Malvern, has yet to draw a paycheck since he opened this BYOB in the summer of 2009. He lives at home with his father to save on expenses. And he's taken on part-time work at Community College of Philadelphia to keep the dream afloat, with just enough tables to keep his prized immersion circulators humming to compress and vacu-cook his white peaches, pork bellies, and lobster tails for "mac and cheese."

As if we needed more proof of Tropea's pluck to use every last resource, imagine the surprise of being given Sonata's "Main Line Restaurant Week" menu - in the urban hipster heart of North Philly!

Well, actually, Sonata is a block beyond the NoLibs artery of Second Street, in a simple Liberties Walk corner space formerly occupied by a forgettable bistro called Swallow. There are also white tablecloths and relative fine-dining prices (mid-$20s), which means that for the collective consciousness of a neighborhood more tuned to its beer bars, bruncheries, and Piazza Jumbotron, Sonata might as well be, well, in Malvern.

But just a few bites into my first meal - a buttery mouthful of flaky house-baked croissant dabbed with fresh fruit conserves, followed by delicately grooved fresh gnocchi clinging to tender shreds of braised lamb - it was clear that Sonata deserved more consideration.

This stark red-and-black glass box of a bistro isn't perfect by a long shot, with a lack of some basic creature comforts - like softer colors and better design, including chairs that don't feel like planks - that has starter restaurant written all over it.

But if ever there was a higher purpose for our BYOB scene, this is it: a showcase for young talent where the food speaks for itself.

And Tropea, aided by his sharp sous-chef Krystal Weaver (once pastry chef at Brasserie Perrier), has been making the most of it, with an ambitious menu of artfully crafted dishes that manage to be at once thoroughly modern in technique and rooted in classic notions of flavor.

His take on seared foie gras, for example, may be one of the city's best, a seared slice of liver over a silky puree of creamed brioche toast, dusted with cashew crumbles and a dollop of black currant preserves that is a startlingly good deconstruction of PB&J. (Tropea credits a dish at Gilmore's for his inspiration.)

His lobster crepes are adorable little bundles of lobster goodness, tender meat in bisquey gravy rolled inside fennel-flecked crepe parcels that are crisped around the edges. His gazpacho, meanwhile, was a vividly fresh homage to early autumn produce, a steeped puree of tomatoes and peppers braced with a vinegar edge, then softened by the vaguely sweet and vegetal chill of cucumber sorbet.

Like many young chefs, Tropea is enamored with sous-vide technology, the "set it and forget it" slow-cook vacuum method that is ideal, in its creative flexibility and preservation virtues, for understaffed kitchens that want to serve a broad menu.

In some cases, it worked wonders, as with the cured pork belly that confits inside a bag for half a day, then arrives crisped atop a smoked tomato marmalade with vacuum-compressed apples and Calvados molasses, a sweet-and-sour effect with a whiff of barbecue. Easily dried chicken also benefits from the high-tech treatment. Wrapped in a paper-thin sheath of crisp bacon, stuffed with ground leg meat and morels (now chestnuts on the fall menu), it tastes like a divinely reengineered chicken sausage that nods to comfort with Boursin-whipped potatoes.

A butter-poached lobster tail is the highlight of Sonata's lobster "mac and cheese." It was satisfyingly decadent, even if the homey title was misleading for a dish closer to a seafood Alfredo, its homemade ribbon pasta coated in fontina-lobster cream.

Tropea's sous-vide experiments with beef were less successful. The short rib, served with celery root puree and orange braised baby carrots, was still pink and too chewy. The sirloin steak, meanwhile, cut into the unfamiliar shape of a tall brick, was just too thick (and slightly overcooked by the grill finish) for the toothy natural chew of the cut. The intense Angus flavor, though, was resoundingly savory.

In fact, focusing flavors was a reliable strength in Tropea's food. He also proved that he could cook without the crutch of gizmos. The pan-crisped cakes of garlicky, cumin-scented mashed chickpeas, sided with yogurt-drizzled frisee and feta cheese, were as satisfying a vegetarian entree as I've seen lately. The scallop crudo, splashed with citrus and the crunchy sparkle of vanilla salt, candied lemon peel, and chile heat, showcased a pristine ingredient.

My favorite, though, was the seared ruby tuna crusted in earthy black trumpet mushrooms. Paired with chanterelles and fava beans, then glazed in Bordelaise gravy enriched with foie gras, the dish was as meaty as any steak - and a perfect match for a lusty Bordeaux.

Tropea's wine-friendly palate hasn't been lost on collectors, whose cult bottles (like the Viader I saw at one friend's table) happily found this BYO haunt long ago.

Don't forget a nice bottle for dessert, too, because Weaver's sweets are worth it. The miniature fig tart could be a still-life. The banana crème brûlée topped with a whipped peanut butter mousse could summon Elvis from the grave. A worthy revamp for the old chocolate trio, though, is the best tribute to Weaver's craft, lining up an especially silky flourless dark chocolate wedge alongside a "chipwich" sandwiching hazelnut gelato and, finally, fresh chocolate beignets.

We closed our lips around those comet-shaped dark fritters, and our eyes snapped open when warm ganache burst into our throats. Don't wait, eat these while they're still warm. And don't hesitate too long to visit Sonata. Someday soon, that bill collector on call waiting may finally get through.

This week, chefs Matt Levin from Adsum and Anne Coll from Meritage join Inquirer restaurant critic Craig LaBan for his live online chat at 2 p.m. Tuesday at http://go.philly.com/phillytalk.


Next Sunday, Craig LaBan reviews Radice in Blue Bell. Contact him at 215-854-2682 or claban@phillynews.com.