1 medium onion
1 large garlic clove
16 ounce can whole tomatoes
2 pounds russet potatoes (about 6 medium)
5 tablespoons olive oil
1 teaspoon minced fresh rosemary leaves
1/3 cup water
1/4 cup freshly grated Parmesan
Cut onion crosswise into 1/2 inch thick slices and mince garlic. Drain tomatoes well and coarsely chop. Peel potatoes and cut crosswise into 1/4 inch thick slices.
Preheat oven to 350 degrees F.
In a 12-inch heavy skillet heat 1 tablespoon oil over moderately high heat until hot but not smoking and saute potatoes in 3 batches, turning them, until golden on both sides, add 1 tablespoon oil for each new batch. Transfer potatoes to a bowl as sauteed.
In skillet cook onion in 1 tablespoon oil over moderate heat, stirring occasionally, until softened. Add onion to potatoes with tomatoes, garlic, rosemary, and salt and pepper to taste and toss to combine well. Transfer potato mixture to a 2-quart gratin dish or other shallow baking dish and drizzle with water and remaining tablespoon oil. Sprinkle Parmesan over potatoes. Gratin may be made up to this point 2 hours ahead and kept, covered, at cool room temperature.
Bake gratin 30 minutes, or until potatoes are tender and beginning to brown. Serves 6.