Melt butter with olive oil in a large enameled Dutch oven or deep skillet over medium heat. Add sliced onions and 1/2 tsp kosher salt. Cook for 20-25 minutes, stirring occasionally, until deeply golden-brown, soft, and jammy.

Stir in minced garlic, fresh thyme leaves, Worcestershire sauce, and black pepper. Sauté for 1 minute until fragrant, scraping up the browned fond from the bottom of the pan.

Pour in the dry rigatoni pasta and 4 cups of beef broth. Stir well to submerge the pasta into the liquid.

Bring to a vigorous boil over medium-high heat. Once boiling, reduce heat to medium-low, cover with a tight-fitting lid, and simmer for 12 to 14 minutes, stirring every few minutes to prevent sticking, until the pasta is al dente and most liquid is absorbed.

Remove the lid. Stir in heavy cream and grated Parmigiano-Reggiano cheese until the sauce becomes silky, rich, and glossy.

Sprinkle the shredded Gruyère cheese evenly over the top. Cover for 2 minutes (or transfer under the oven broiler on high for 2-3 minutes) until the cheese is completely melted, bubbling, and lightly golden.

Garnish with fresh thyme leaves and freshly cracked black pepper. Serve piping hot with toasted garlic sourdough!