In a heavy pot or Dutch oven, melt butter and olive oil over medium heat. Once shimmering, add onions, seasoning with salt and pepper. Cook for about 45 minutes until golden brown.

Sprinkle in brown sugar, stirring it into the onions for 2 minutes to enhance sweetness.

Pour in white wine (if using) and scrape browned bits from the pot. Let it simmer until the wine reduces slightly.

Add broth, thyme, and bay leaves to the pot. Stir and bring to a gentle simmer for 15 minutes.

Preheat the oven to 425°F (220°C).

Stir in gnocchi into the simmering broth, cooking undisturbed until they float, about 3-4 minutes. Remove thyme stems and bay leaves.

Mix in Gruyère and Parmesan cheese until melted. Pour into a baking dish and top with remaining Gruyère.

Bake for 15-20 minutes until cheese is melted and golden brown.

Let rest for 5 minutes, then garnish with fresh thyme leaves. Serve warm.