Place the bacon in a large non-stick skillet and set over medium heat. Cook until the bacon is crisp, but not overly dark. Drain away all but 1 tablespoon of the bacon fat. Add the shallots and leeks and cook until tender, about 5 minutes. Cool the mixture to room temperature. Preheat the oven to 350 degrees F. Place the eggs, milk, cream, salt and pepper in a bowl and whisk to combine. Spoon half the bacon/onion mixture into the bottom of the pie shell; sprinkle with half the cheese. Pour in the egg mixture and top with the remaining bacon/onion mixture; sprinkle with the remaining cheese. Bake for 40 to 45 minutes, or until the eggs are set. Rest the quiche for 5 minutes to allow the filling to set before slicing and serving.