1. Separate the white from the green part of the leek.
2. Remove the outer leaves of the white part, wash it.
3. Cut it into 4 to 5 centimeter pieces.
4. Precook them in steam (keep them slightly crunchy).
For the béchamel sauce: Prepare the white roux: in a saucepan, melt the butter over low heat, add the flour.
5. Mix until obtaining a homogeneous mixture and cook for 3 minutes over low heat (without browning).
6. Let it cool.
In another saucepan, bring the milk to a boil.
7. Pour it gradually over the cooled white roux.
8. Mix using a whisk.
9. Bring to a boil for about 5 minutes while stirring.
10. Season.
11. The sauce should thicken gradually, becoming creamy and coating. Cut the brie into cubes and add it to the béchamel sauce.
12. Let it melt. Butter an oven dish, place the leek pieces next to each other.
13. Cover with sauce and place under the oven grill until the surface is gratinated.