1. Preheat the oven to 300F.
2. Chop the leeks.
3. In a saucepan, heat the butter and add the leeks. Cook for 2 minutes over medium heat. Cover and cook over low heat for 6 minutes. Remove from heat.
4. In a bowl, whisk the milk and eggs. Season generously, add the flour and cheese, whisking. Add the leeks. Mix.
5. Butter a mold.
6. Pour the preparation into the mold. Place the mold in a deep baking dish. Pour water up to half of the mold.
7. Bake for 35 minutes.
8. Remove from the oven. Let cool for 5 minutes before unmolding. Serve hot, warm, or cold.