1. Using a small pointed knife, scrape the mussels and debeard them under cold water.

2. Peel the onion and carrot; finely chop. Clean the endives and mushrooms; finely chop.

3. In a saucepan, melt 100 g of butter; sauté the garlic, onion, and carrot. Add the white wine and bouquet garni.

4. Add the mussels; cook over high heat until they open (5 to 6 minutes).

5. Remove the saucepan from the heat; remove the mussels and strain the cooking liquid. Shell the mussels and set aside.

6. In another saucepan, heat 50 g of butter, add the endives, and cook over low heat for 10 minutes. Add the mushrooms, beer, and some of the mussel cooking liquid; continue cooking to reduce. Set aside.

7. In a saucepan, melt 50 g of butter; add the flour and mix. Whisking, gradually add the mussel cooking liquid; add the cream.

8. Mix this sauce with the mussels, endives, and mushrooms. Divide into gratin dishes; cover with grated Gruyère and place under the oven grill for a few minutes.