Golden Onion Casserole

Name: Golden Onion Casserole
Ingredients: 6 medium yellow onions thinly sliced; 1/2 cup butter divided; 1/4 cup flour; 1/2 teaspoon salt (optional); 1/4 teaspoon white pepper; 2 cups whole milk at room temperature; 2 tablespoons chicken bouillon granules (use 1 tablespoon if using double-strength cubes); 1/2 cup sherry (or Burgundy if serving with red meat); 3/4 cup (total) mixed Gruyere and Swiss cheeses shredded; 1 French bread or onion-garlic baguette cut in thin slices; 4 tablespoons melted butter