1. Open the scallops and remove the meat and the coral. Wash them and place them on absorbent paper to drain.
Once dry, cut them into cubes, leaving the coral whole. Set aside.
2. Meanwhile, thoroughly wash 6 empty shells. Set aside.
3. Finely chop the leek and onion.
4. Melt the butter in a saucepan and lightly sauté the vegetables, stirring with a wooden spoon. Let simmer for a few minutes.
5. Add the flour and mix. Pour in the white wine and let cook for about 10 minutes over low heat.
6. In a bowl, mix the heavy cream and egg yolks, then incorporate this mixture into the saucepan off the heat. Season.
7. Divide the scallop meat into the shells. Cover them with the vegetable sauce and sprinkle with grated gruyere cheese.
8. Bake and let gratin for 6 to 7 minutes in a very hot oven at 475°F (240°C).
Serve piping hot.