It's always a thrill to witness a daring young chef make his first splash at a surprising new restaurant. But how many ever mature enough to begin to approach greatness?

The club is understandably elite. And while it's difficult enough to arrive there by staking out fresh territory with an original restaurant, it is perhaps an even more impressive feat to do it by inheriting - and improving - a legend's swan song.

That is especially true when the legend is Jean-Marie Lacroix, the snowy-haired gentleman chef whose posh finale - Lacroix at the Rittenhouse - capped a two-decade career at the Four Seasons Hotel, where he built the Fountain into an institution. His namesake has been one of the city's most beautiful dining spaces, an airy room padded with green velvet chairs that hovers tranquilly above the park like a rarefied culinary oasis. But it had yet to truly reach its lofty potential before Lacroix, 66, eased into retirement last summer.

The arrival of new chef Matthew Levin, however, is proving to be just the energy jolt that Lacroix the restaurant needed. The 34-year-old Philadelphia native is a Culinary Institute of America grad and a veteran of Striped Bass, Le Bec-Fin and Aureole, who first starred at New Hope's Moonlight with contemporary pyrotechnics like tempura-fried lobster tails sparked with pop rocks.

That gastronomic adventurism has evolved considerably over the course of his subsequent posts at the Bridgetown Mill House Inn, and most recently central Jersey's Pluckemin Inn. And the vision he has brought to Lacroix is ripe, revealing an artful command of rare ingredients and cutting-edge techniques cast onto the plate with startling creativity.

It's obvious with stunning dishes such as butter-poached Scottish langoustines with lemon and licorice root puree and butterbeans tossed in grilled black olive vinaigrette. Or the Cuban pork belly - braised for 24 hours, its top broiled to a delicate crisp - that evaporates in the mouth like savory custard ribboned with tender meat. Or even a garnish of golden beets and asparagus tips tossed in gouda cream, an amber froth of vermouth and aged cheese so diabolically good it should be served on its own with a straw.

Of course, running a grand hotel kitchen, with its myriad menus and endless hours, is an awesome task that goes far beyond one chef's personal expression. And in many ways, Levin has shown maturity by knowing when to simply refine and polish what already worked.

Chief among them is chef Lacroix's blowout Sunday brunch, which redefined the weekend meal as the ultimate small-plate orgy. The normally sedate room comes alive in the warming Sunday light with a palpable glamour, for which all of Rittenhouse Square, it seems, shows up in its finest glitz. For a mere $52, there is a jaw-dropping array of exquisite hors d'oeuvres and edible jewels that stretch from the hostess stand deep into the kitchen, where chefs slice grilled Vietnamese boar at the roasting board, and there's an entire station of exotic gratins (sweetbread and wild mushrooms? Tuscan bread pudding? Truffled purple cauliflower? How to choose? Take them all!!)

It's hard to save room after platefuls of oysters, caviar with fresh blinis, and giant shrimp dunking in cocktail-sauce shots. I downed spoonfuls of eggplant caviar glazed with smoked chile oil, and smoked salmon mousse with avocado panna cotta. There were glasses of cauliflower and lobster cream sprouting tempura-fried asparagus spears, salad bowls filled with heirloom beans, and charred scallions with truffled maitake mushrooms.

And then, of course, there was pastry chef Fred Ortega's symphonic buffet of exquisite mini-desserts, a chocolate fountain with homemade marshmallows, and a basin of liquid nitrogen that deep-froze poufs of curried coconut milk foam into crackly meringue clouds filled with Cointreau jelly.

I ate salad for dinner.

While brunch has always been a four-bell feast, Lacroix at the Rittenhouse was rarely able to put its food and service together with any consistency at lunch or dinner until now.

The service - always outgoing but often out of sync in previous years - finally found harmony during each of the meals for this review. Our splendidly pleasant server, Jorge Sanchez, never let a napkin go unfolded, or a push-pot of robust coffee get tepid on our many trips to the brunch buffets. And sommelier Eric Simonis loosened up nicely when challenged to find something beyond the cellar's prestige French bottles - and he succeeded in mining some nice gems from Chile (a cabernet franc from Valdivieso), and a quenchingly dry Alsatian muscat ottonel from Barmes-Buecher.

At each meal, however, the kitchen was the undeniable star. Even the French fries at lunch - the beneficiaries of a three-day treatment involving duck fat and truffle oil - were unbelievably good.

Levin's most elaborate efforts, though, are at night, where the chef has added a handy a la carte option to the existing tasting menu. Among the highlights was an unusual "gratin" of rich mussel broth studded with tender mollusks, salty nuggets of guanciale, and pillows of gnocchi that melted away like dumpling wisps. A magnificent rib steak, sliced through the midriff and splayed like two medium-rare meat Frisbees, sparkled with a zesty Asian crust of toasted nori, sesame seeds, and bonito-scented sea salt.

The multicourse tasting meals, meanwhile, are a showcase for sharp concepts and prized ingredients - like the season's first Nantucket Bay scallops (a tiny harvest Lacroix divvied up with Thomas Keller at Per Se and Alex Stratta in Vegas.) Levin served them pristinely, but with a tweak, the sweet raw scallops shined in prime olive oil and citrus vodka, then scattered with the tart crunch of tiny diced apples and the salty pop of caviar.

A thick fillet of black bass came with a "cannelloni" of minced shrimp wrapped in a tempura-fried seaweed tube, and a pedestal of lotus root stewed with mulled molasses and chestnuts. Raw squid was sliced into toothy seviche-style "noodles" illuminated by a marinade ringing with the zest of grated Buddha Hand citrus. Meanwhile, the intensity of kobe beef crusted with Japanese spice was magnified by a concentrated schmear of citrusy peanut butter.

Among the most startlingly good creations, though, was Levin's seafood "latte." This intense cup of roasted shellfish bisque, steeped with lemongrass and ginger, came topped with a dollop of coffee-scented milk foam, a twinkle of gold leaf, and a sidespoon cradling a nugget of salt-speckled toffee.

Stirred together, the already intriguing soup transformed into an otherworldly potion of sweet deep-sea luxury with a whiff of smoky salt. It was at once profoundly complex, ethereally light and totally unexpected. So make that latte a grande next time. Because it's the taste of greatness brewing.

Contact restaurant critic Craig LaBan at 215-854-2682 or claban@phillynews.com. Read his recent work at http://go.philly.com/craiglaban.