1. Split the leeks in half and cut them into thin slices. Melt the butter in a sauté pan, add the leeks and mix well. Cover and cook over low heat for 15 to 20 minutes.
2. Preheat the oven to 180°C (350°F).
3. Meanwhile, unroll the pie crust in a large tart pan. Sprinkle with grated gruyère cheese.
4. In a bowl, mix the eggs, milk, cream, salt, and pepper.
5. Place the leeks in the bottom of the tart on top of the gruyère cheese and cover with the egg/cream mixture.
6. Bake for 35 minutes.
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