A gratin that highlights potatoes and tomatoes

The gratin is one of those classic dishes that is both a vessel and a method of cooking — a shallow baking dish is filled with seasonal vegetables that are slowly baked to become meltingly tender and lightly crisped.

The traditional potato gratin is an über rich and heady mélange of potatoes, butter, cream and cheese, an all-time favorite. But in southern France, tomatoes are layered in with the potatoes to contribute their natural juices and sweet/tart flavors. This brightens the mix and lightens the role played by cream. A Provençal potato-tomato gratin bridges the seasons — the last of summer's tomatoes mingle with fresh potatoes just pulled from the ground.

Fresh potatoes sport thin, wispy skins and a crisp waxy texture. They are unlike those that have been "cured" or stored for a couple of weeks to toughen up the peel and dry the flesh.

Fresh potatoes can be any size, ranging from marbles to bakers.

To determine a potato's freshness, rub the skin with your thumb. It should be delicate enough to scrape clean. Look for fresh potatoes, now in our farmers markets, that are smooth without nicks or scrapes. Because they're not "cured," they won't last long, several days rather than weeks. Store at room temperature and don't refrigerate because their starches will begin to convert to sugar and they'll taste sweet.

Wait until ready to cook before washing because moisture expedites must and mold. Lightly scrub with a brush, then rinse quickly under running water. Once cut, use them as soon as possible. as they tend to turn brown if exposed to air for too long. Fresh potatoes are especially moist and perfect for the long, slow cooking a good gratin requires (and unlike store-bought potatoes, do not need to be precooked).

Using up the last of the season's tomatoes, both the ripe and the green, is immensely satisfying. Those that are still a bit hard, not fully red, need heat to become worth eating. When slow baked, they take on the flavors of their more mature cousins. Of the hundreds of different types of tomatoes, I prefer the cherry or Roma best for gratins because they're juicy and meaty, but any fresh, local tomato will work equally well.

Satisfyingly simple, this dish is perfect for entertaining for it may be assembled and baked ahead and gently reheated before serving. Yet, it's so homey and comforting, it's just right any night of the week.

Beth Dooley is the author of "In Winter's Kitchen." Find her at bethdooleyskitchen.com.