Prepare the tart shell. Roll out the dough to an 11-inch circle on a lightly floured surface. Transfer to a 9-inch tart pan with a removable bottom and trim the edges. Refrigerate for at least 30 minutes.
Preheat oven to 350 degrees F.
Cut the leeks in half lengthwise and then crosswise into 1/4-inch thick half circles. Place in a bowl and rinse well in 2 changes of water. Drain.
Line the pastry with parchment paper and fill with pie weights or dried beans. Bake until the crust is set, about 12 minutes. Remove the paper and weights and bake until golden brown, 8 to 10 minutes. Remove from the oven and cool on a wire rack.
In a medium skillet, cook the bacon over medium-high heat until crisp and the fat is rendered, about 5 minutes. Remove and drain on paper towels. Add butter to the fat remaining in the pan. When foamy, add the drained leeks, garlic, salt, pepper, and nutmeg, and stir well to combine. Reduce the heat to low, cover, and cook until the leeks are very soft but not browned, 15 minutes. Remove from the heat and drain in a fine mesh strainer.
Combine the eggs and the cream in a medium bowl and whisk well. Add the bacon, Parmesan, and Gruyere, and whisk well. Add the leeks and whisk to combine. Pour into the chilled tart shell and bake risen and golden brown, 40 to 45 minutes.
Remove from the oven and cool for at least 20 minutes on a wire rack before serving.
Yield: 8 servings