Heat olive oil and butter in a large pan
Add sliced onions and cook, stirring occasionally, until golden and caramelized
Deglaze the pan with cognac
Add sherry vinegar, Worcestershire sauce, and beef stock
Reduce the sauce until it thickens
Stir in the heavy cream
Cook spaghetti until al dente
Transfer cooked spaghetti directly to the sauce
Add some starchy pasta water and let everything come together
Garnish with freshly grated parmesan and fresh herbs
