Line tart pan with puff pastry sheet, top with foil and pie weights, and prebake according to package directions. Let cool on a wire rack and set aside. Preheat oven to 425.
Trim leeks down to light green part. Slice in half lengthwise and then slice very thinly. Add to a colander set over cold water in a bowl and swish to remove sand. Lift leeks out and drain; repeat twice, using fresh water. Thinly slice onions.
Add bacon to a heavy Dutch oven over medium heat and cook until crisp, set aside to drain; reserve 2 tablespoons of bacon fat. Add butter and sautée leeks and onions, cooking until lightly browned. Use some salt to cook the vegetables down, about 20 – 25 minutes.
Make custard by mixing eggs and cream. Fill baked tart shell with bacon pieces, leek mixture and season with salt & pepper. Pour in custard and top with cheese.
Bake at 425 degrees for 40 minutes.
Can be frozen before baking.