Caramelize the Onions: In a large Dutch oven or deep skillet, melt butter with olive oil over medium heat. Add thinly sliced onions with 1/2 tsp kosher salt. Cook, stirring occasionally, for 20 to 22 minutes until deep golden-brown, sweet, and jammy. If they brown too fast, lower heat to medium-low and splash with 1 tbsp of water.

Aromatics & Deglazing: Stir in minced garlic and fresh thyme; cook for 1 minute until fragrant. Pour in the white grape juice-vinegar mixture to deglaze, scraping up all the savory caramelized browned bits (fond) stuck to the bottom of the pot.

Simmer Pasta in Broth: Pour in the beef broth and Worcestershire sauce. Bring to a rolling boil. Add dry uncooked pasta and press down so it is fully submerged. Reduce heat to medium, cover loosely, and simmer for 10 to 12 minutes, stirring every couple of minutes, until the pasta is al dente and most of the liquid is absorbed into a luscious starchy sauce.

Cream & Cheese Velvet Finish: Turn off heat. Stir in heavy cream until velvety and glossy. Fold in 1 cup of shredded Gruyère and all of the grated Parmesan cheese until completely melted and smooth.

Broil or Melt Top: Scatter the remaining 1/2 cup Gruyère cheese across the top. Cover with lid for 2 minutes to melt (or pop under the oven broiler for 2 to 3 minutes until bubbling and blistered golden). Garnish with chopped fresh parsley and extra thyme, then serve immediately.