Heat olive oil and butter in a large pan
Add sliced onions and cook until caramelized, seasoning with salt and black pepper
Add fresh thyme sprigs to the onions
Deglaze the pan with cognac
Add sherry vinegar and Worcestershire sauce
Pour in beef stock and simmer
Stir in heavy cream
Cook spaghetti according to package directions and drain
Combine spaghetti with the French onion sauce
Garnish with freshly grated parmesan and fresh herbs before serving
