1. Preheat the oven to 375°F.
2. In a saucepan, cook the mussels in the vegetable broth for 2 minutes covered to open them. Drain. (Keep the cooking broth for later use.)
3. Open the mussels in half and keep the mussel in a half-shell.
4. In a gratin dish, pour a drizzle of oil. Place the half-mussels, shell down. Season with pepper. Place the grated Gruyère on each mussel, being careful not to overflow.
5. Bake to gratin for 3 minutes.
6. Remove from the oven and serve with a rice pilaf or a salad.