In a large skillet or Dutch oven, melt the unsalted butter over medium-low heat.

Add the sliced onions, kosher salt, and ground black pepper. Cook slowly, stirring occasionally until caramelized—about 35-45 minutes.

Stir in minced garlic and fresh thyme. Cook for about 1 minute, or until fragrant.

Pour in the dry white wine and sherry vinegar, scraping up any flavorful bits stuck to the bottom.

Push the onions to one side, then add the rigatoni. Pour in the beef broth to submerge the pasta.

Increase the heat to high until it’s simmering. Cover and let it simmer for about 15 minutes or until the pasta is al dente.

Stir in the Gruyère and Parmesan cheese until melted and creamy. Add half-and-half for extra richness and adjust seasoning.

Garnish with more fresh thyme or cheese before serving to enhance the flavor.