Heat oven to 400° and unroll puff pastry sheet on a parchment paper lined baking sheet. Carefully poke holes in puff pastry, without going all the way through to the pan. Cook in oven until puffed up and lightly browned, 10 minutes. After, carefully push air out making the sheet flat again.
In a small pot over medium heat, melt 1 Tbsp of butter. Add leeks and onion, stir to coat in butter. Add thyme, salt, and pepper. Stir, and cover. Let cook for 20 minutes, stirring frequently. Add remaining 1/2 Tbsp butter and continue cooking another 15 minutes, stirring frequently until leeks and soft and browned (almost like an onion jam!)
In a separate small pan, over medium - high heat, add chopped bacon and cook, stirring frequently, until bacon is crispy and brown, 7 - 10 minutes. Remove from pan and let sit on paper towels to drain excess grease.
Layer the leek mixture onto the puff pastry sheet, leaving a crust. Sprinkle with bacon, and gouda cheese. Place back in over for another 5 - 7 minutes to melt cheese.
