Bring a large pot of slightly salted water to a boil. Cook ravioli in the boiling water for 8-10 minutes, or until done. Drain and set aside.  Preheat oven to 350°F.

In a 12-inch cast-iron skillet, melt butter with olive oil over medium-low heat.

Add onions and garlic; cook, stirring occasionally, until softened and lightly browned, about 25 minutes. Stir in flour; cook, stirring occasionally, for 5 minutes. Stir in broth, thyme, salt, and pepper; add in the meatballs and bring to a boil. Reduce heat; simmer until slightly thickened, about 20 minutes. 

In the cast-iron skillet cover the bottom of the skillet with half of the sauce. Toss the cooked ravioli with the remaining sauce and arrange in the casserole. Top with cooked meatballs. 

Preheat oven to broil. Sprinkle cheese onto meatballs.

Broil 6 inches from heat until cheese is melted and lightly browned, 3 to 5 minutes. Garnish with thyme, if desired. Serve immediately.