Bring a pot of salted water to a boil and cook your chosen pasta according to package instructions. Drain and set aside.

In a large skillet over medium heat, combine butter and olive oil. Allow the butter to melt and blend well with the oil.

Add thinly sliced onions with a pinch of salt. Cook for 15-20 minutes until soft and deeply caramelized.

Stir in sugar and continue cooking for about 5 more minutes.

Pour broth into the skillet and bring to a simmer. Cook for 5-10 minutes, seasoning with salt and pepper.

Gently toss the cooked pasta into the onion mixture until evenly coated.

Preheat your oven broiler. Transfer the mixture to an oven-safe dish and sprinkle Gruyère and mozzarella cheese on top.

Broil for 2-3 minutes until the cheese is bubbling and golden.

Allow to cool slightly, then garnish with fresh thyme before serving warm.