Roll out the dough to an 1/8-inch thickness on a lightly floured surface. Using a 3-inch cookie cutter cut out 48 rounds of dough. Press the dough into the wells of a mini muffin tin (the standard mini muffin tin has 24 wells) 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.

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. Spoon about 2 teaspoons of the mixture into the chilled puff pastry dough and bake until the dough has risen and the tartlets are golden brown, 20 to 25 minutes.

Remove from the oven and cool for at least 20 minutes on a wire rack before serving.