In an oven-safe skillet, melt butter and olive oil over medium-low heat. Add sliced onions and cook for about 25–30 minutes until caramelized.

Stir in minced garlic, fresh thyme, salt, and pepper. Cook for 1 minute. Add balsamic vinegar to deglaze the pan.

Add dry orzo pasta and beef broth to the skillet. Bring to a simmer and cook uncovered for about 8–10 minutes until orzo is tender.

Lower the heat and stir in half of the Gruyère and mozzarella cheese until melted. Top with remaining cheese.

Preheat oven to 400°F (200°C) and bake the dish for 10–15 minutes until the top is golden brown.

Remove from the oven, let cool slightly, and garnish with parsley or chives. Serve warm and enjoy.