Preheat the oven to 400°F (200°C).

Slice the leeks thinly.

In a saucepan, add the butter and leeks. Cook over low heat, stirring frequently, for 20 minutes until softened. Season to taste.

While the leeks are cooking, prepare the filling in a bowl: combine the cream, milk, eggs, salt, and pepper. Mix well.

Butter and flour a tart pan, then line it with the puff pastry sheet.

Sprinkle the breadcrumbs evenly over the pastry.

Spread the cooked leeks over the bottom of the tart.

Slice the brie and arrange it on top of the leeks.

Bake for 30 minutes.