Melt olive oil and butter in a large pan or Dutch oven. Start by caramelizing the yellow onion. Add thinly sliced yellow onion. Stir and cook on medium heat. Continue to cook stirring frequently until the onion is caramelized. Depending on the thickness of the sliced onion and the pan used time taken to caramelize the onion can vary. It took me 20 minutes for the onions to caramelize.

Once onion is caramelized add minced garlic and sliced mushrooms. Stir and cook for a minute.

Deglaze pan with dry white wine and Worcestershire sauce.

Add plain flour, stirring constantly, and cook for 1 to 2 minutes until the flour is fragrant.

Pour the stock into the pan. Add thyme leaves, salt, and pepper to taste.

Once stock begins to boil add pasta.

Cover and cook pasta, stir it often. Cook until al-dente.

Remove thyme leaves and discard.

Add half and half and sherry vinegar. Stir until blended.

Remove pan from heat.

Sprinkle little fresh thyme and fresh grated Parmesan cheese.

Serve immediately!