By Paul Reidinger

PHYSICS STUDENTS WILL have struggled with Heisenberg's uncertainty principle, but where there is uncertainty there must also be certainty, and that means, if you step into Canteen, you are sure to see Dennis Leary. This is in part because Canteen is quite small – a narrow adjunct to the lobby of the Commodore Hotel – and in larger part because Leary is the owner and chef. He also waits tables as needed and sees to the provisioning. If he were a musician, he'd be one of those guys who sits at a trap set with a harmonica mounted in front of him and a trumpet in one hand: a one-man band, in other words. And if you ever doubted that such a model could succeed in the restaurant business, you have only to glide into Canteen to be reassured.

Leary was, until recently, the chef at Rubicon – a Financial District restaurant whose kitchen has seen its share of big names, including Traci des Jardins and Elizabeth Falkner. But while des Jardins and Falkner moved on to other pastures green with the color of money (Jardinière, Acme Chophouse, Citizen Cake), Leary seems to be conducting a back-to-the-roots, labor-of-love experiment. It is wise to take these sorts of chances when one is young; Leary is 34, and while I reckon that age to be well within the bounds of youthfulness, it is true at the same time that youth is relative (according to a theory of relativity?), and it is relative not least to the burdens it is asked to carry. Shopping, prepping, plating, waiting tables, answering the phone, and dealing with nettlesome business matters is a dawn-to-dusk, constantly-on-your-feet, iron-man array of tasks that could make a teenager weary. Yet Leary (who looks younger than his years) does not lack for energy and enthusiasm, and what he offers at Canteen is an intimacy of homemade-ness that is as near an approximation to a private dinner party as you are likely to find in any restaurant in the city.

The space still shows ample evidence of its diner past: It is quite deep and tight, with a long counter running along one wall and a line of booths along the other. The booths present some ergonomic issues, and Leary has plans to deal with them. He's also planning to lay a new floor (the incumbent is a wasteland of dreary blue linoleum tiles) and freshen the slightly wan yellow paint. Alas, the light fixtures suspended over the counter are on their way out too, despite their protuberance-rich resemblance to spy satellites, or perhaps strange creatures of the deep, cast in glowing blue glass.

Although Canteen's threadbare-hip look does not suggest Gallic influences, Leary is well schooled in classic French technique. On a recent evening, damp with winter, he turned out a gorgeous and sustaining confit of duck leg (the main event in a three-course, $32 fixed-price menu), which was crisp and juicy – the way many of us wish our Thanksgiving turkeys had turned out. (Memo to self: next year, try turkey confit!) Each whole leg (drumstick and thigh) was nestled on a gentle bed of braised fennel shavings punctuated with some halved quince to remind us that this is California and California has its seasons too. (Quince is a winter fruit.) Leary preceded the confit with a simple crab soup (with chunks of crabmeat, another of winter's delicacies, adrift in a clear broth instead of a bisque or puree) and followed it with little chocolate pots de crème still warm from the oven and of an almost pornographic gooiness.

At the moment, though, the heart of the action at Canteen is of the breakfast and lunch variety. (Leary hopes soon to offer a steady dinner service, and his beer-and-wine license is pending, and plainly these variables belong in the same equation.) The daytime food can be downright homey, but even a red-state dish like corned beef hash ($8.95) is made with brisket Leary has corned himself, while a lovely amuse-bouche of butternut squash soup is softened to the pale yellow of sabayon by the subtle incorporation of celery root. The menu is stable enough to be printable, but at the same time one suspects Leary is constantly adjusting and improvising to reflect (farmers) market conditions. It is a truism that restaurants succeed by offering the same dishes endlessly and changelessly – the somber miracle of mass production – but it might be that Canteen will succeed by inverting that truism, by never offering quite the same thing twice.

My friend described his smoked-salmon omelette ($7.50), swollen with cream cheese, as "a deli omelette." Yes: an omelette filled with shmear; the omelette as bagel. It was served with buttered whole-grain toast and a side of home fries ($2.50) that seemed to have been sautéed instead of deep-fried and were still slightly steaming with moistness. (Frying is a form of drying, and deep-frying is a far more intense procedure than sautéing, so do the math.) Leary had, of course, smoked the salmon himself, though I suspect he had not baked the whole-grain bread nor churned the butter. He is a can-do guy, but even a can-do guy shouldn't have to do everything. Canteen. 817 Sutter (at Jones), S.F. (415) 928-8870. Mon.-Fri., 7 a.m.-2 p.m.; Sat.-Sun., 8 a.m.-3 p.m. Beer and wine license pending. American Express, Diners Club, Discover, MasterCard, Visa. Pleasantly noisy. Wheelchair accessible.