Cook Onions In a 4- to 6-quart dutch oven heat the butter over medium. Add the thinly sliced onions and a generous pinch of salt. Cook, covered, 5 minutes. Uncover and reduce heat to medium-low. Cook, stirring frequently, until caramelized, about 30 minutes.
Add Flavorings Add the garlic and fresh thyme. Cook for 2 minutes or until the garlic is fragrant. Add the white wine to deglaze the pan, scraping up any browned bits.
Add Beef Stock and Pasta Add the beef stock to the pot and bring to a boil. Stir in the pasta and cook, uncovered, until al dente, about 12 minutes. Remove the pot from heat and discard the thyme sprigs.
Finish and Serve Stir in the heavy cream, shredded cheese, and sherry vinegar; stir until well combined. Season to taste with salt and black pepper. If desired, top with additional shredded cheese and fresh thyme.
