• 1 pound red-skinned or yellow-fleshed potatoes
• 1 large celery root
• juice of 1 lemon
• salt
• 1 to 3 tablespoons virgin olive oil
• 1 medium-sized red onion, quartered and sliced crosswise
• 2 garlic cloves, finely chopped
• 2 small celery stalks, finely diced
• 1 tablespoon chopped basil or 1 teaspoon dried
• coarsely ground black pepper
• 1 cup dry white wine, stock or cooking water from the potatoes
• 1 pound ripe tomatoes or 1 15-ounce can whole tomatoes, seeded and cut into large pieces, juice reserved
• thinly shaved Parmesan, dry Monterey Jack or dried goat cheese (optional)

Scrub the potatoes and, if the skins look firm and fresh, leave them on; if not, peel them. Slice the potatoes into rounds just slightly more than ¼ inch thick. Peel the celery root; slice it into quarters and then into pieces the same thickness as the potatoes. As you work, put the celery root in a bowl and cover with cold water and the lemon juice.

Bring several quarts of water to a boil. Add salt to taste and the potatoes and cook until they are tender but not yet done, about 5 minutes. Remove them; then add the celery root and cook until tender but still firm, about 3 or 4 minutes. Remove. Reserve the water.

Warm the oil in a frying pan and add the onion, garlic, celery, basil, and season with salt and pepper. Cook gently over medium heat until the onions have begun to soften, about 5 minutes. If the pan becomes dry, add either a little more oil or a few tablespoons water. When the onions are soft, add the wine, raise the heat, and reduce the wine by about half; then add the tomatoes and their juices plus ½ cup of the cooking water from the potatoes. Simmer for 4 or 5 minutes.

Preheat the oven to 375°F. Lightly film an 8- by 10-inch gratin dish with oil. Ladle half the tomato sauce in the bottom of gratin dish, make a layer of alternating pieces of potato and celery root, and season with pepper. Repeat, using the rest of the potatoes and celery root; then cover with the remaining sauce. Cover and bake until the potatoes and celery root are completely tender, about 45 minutes. Remove the gratin from the oven and let it settle several minutes before serving it. As it cools a little, the flavors will emerge.

Serve this, if you like, with thin shavings of dry cheese on top, such as Parmesan, dry Monterey Jack, or a dried goat cheese.