1 recipe Pate Brisee or pie crust of your choice
12 small leeks (about 3 pounds) or a mixture of 2 pounds leeks and 1 large onion
4 tablespoons unsalted butter
1/2 teaspoon salt
Freshly ground black pepper
2 large eggs
1/4 cup creme fraiche or heavy cream
3 to 4 ounces sliced Parma ham (prosciutto), coarsely chopped
1 cup freshly grated imported Gruyere cheese
Prepare the pastry shell: Roll out the dough to line a 9-inch tart pan. Carefully transfer the dough to the pan. Chill for 30 minutes, or until firm.
Preheat the oven to 425 degrees F.
Prepare the filling: Trim the leeks at the root. Cut off and discard the fibrous, dark green portion. Split the leeks lengthwise for easier cleaning, and rinse well in cold water until no grit appears. Coarsely chop the leeks. If using an onion, coarsely chop it also.
Melt the butter in a medium-size saucepan over low heat. Add the leeks and onion if using, salt, and pepper to taste and cook, covered, until the leeks are very soft but not browned, about 20 minutes. If the leeks have given up an excessive amount of liquid, drain them in a colander.
Combine the eggs and cream in a medium-size bowl and mix until throughly blended. Add the leeks and mix again. Reserve 1/4 cup each of the ham and the cheese to sprinkle on top of the tart. Mix the rest into the leek mixture.
Pour the leek mixture into the prepared pastry shell. Sprinkle with the reserved ham, and then the cheese. Season generously with freshly ground black pepper.
Bake until nicely browned, 40 to 45 minutes. Serve warm or at room temperature. 6 to 8 servings.