Preheat oven to 425°F.
Place the puff pastry in a pie pan. Be sure to line with foil or baking parchment to stop the sides from collapsing. Place into the oven to bake blind, about 5 minutes, or until it is golden brown.
In a sauté pan, cook the leeks and zucchini in butter until they're soft.
Once the pastry is golden brown, remove from oven and decrease the temperature to 375°F. Remove the lining from the pastry; add the cooked vegetables and Cheddar.
Whisk together the eggs and milk, add salt and pepper to taste. Carefully pour the mixture into the pie pan over the vegetables and cheese.
Bake at 375°F for 15 to 20 minutes, or until the quiche is set.