French Onion Baked Lentils and Farro

Name: French Onion Baked Lentils and Farro
Cuisine: European, Italian, Mediterranean, French
Ingredients: 2 pounds (910 grams) yellow onions (4 large or 5 medium), sliced; 2 tablespoons (30 grams) unsalted butter; 1 tablespoon (15 grams) olive oil; Kosher salt and freshly ground black pepper; 2 tablespoons (30 ml) dry sherry, vermouth, or white wine (optional); 2 cloves garlic, minced; 1 1/2 cups (300 grams) dried farro; 3/4 cup (135 grams) dried lentils, any variety, but green (le puy) or brown (italian) are my favorites here; A few sprigs of fresh thyme; 6 cups (1.4 liters) vegetable broth + splashes more, if needed; 2 cups (8 ounces or 225 grams) coarsely grated gruyere cheese