French Onion Stuffed Shells

Name: French Onion Stuffed Shells
Ingredients: About 2 pounds yellow onions, preferably large; 5 tablespoons olive oil, divided; 30 dried jumbo pasta shells (about 8 ounces); 6 ounces Parmesan cheese (or 2 cups store-bought grated), divided; 8 ounces Gruyère cheese; 1 medium bunch scallions; 4 tablespoons (1/2 stick) unsalted butter; 1/4 cup all-purpose flour; 1 1/2 cups whole milk; 1 1/2 cups beef broth; 2 teaspoons Worcestershire sauce; Salt; 3 large cloves garlic; 6 sprigs fresh thyme, divided; 1 (15 to 16-ounce) container ricotta cheese (about 2 cups); 1 1/2 teaspoons freshly ground black pepper, divided; 1/2 cup bread crumbs