Heat butter and olive oil in large pot over medium heat.
Add sliced onions and thyme, cook slowly for 1 hour stirring occasionally until golden brown and caramelized.
Add garlic and cook 1-2 minutes until fragrant.
Pour in white wine and scrape up browned bits, simmer 2-3 minutes to reduce.
Add Worcestershire sauce, salt, and pepper.
Pour in beef broth and add dry pasta, bring to boil then reduce to simmer.
Cover and cook 12-15 minutes stirring occasionally until pasta is al dente.
Reduce heat to low and stir in heavy cream, sherry, and grated Gruyère until cheese melts and sauce is creamy.
Taste and adjust salt as needed.