French Onion Baked Lentils and Farro

Name: French Onion Baked Lentils and Farro
Cuisine: European, Mediterranean, French
Ingredients: 2 pounds yellow onions (4 large or 5 medium); 2 tablespoons unsalted butter; 1 tablespoon olive oil; Kosher salt and freshly ground black pepper; 2 tablespoons dry sherry, vermouth, or white wine (optional); 2 cloves garlic, minced; 1 1/2 cups dried farro; 3/4 cup dried lentils, any variety but green (le puy) or brown (italian) are my favorites here; A few sprigs of fresh thyme; 6 cups vegetable broth + splashes more, if needed; 2 cups (8 ounces) coarsely grated gruyere cheese