In an 11-inch skillet, heat 2 tablespoons of olive oil and 2 tablespoons of salted butter over medium heat until melted and bubbly. Add the 2 lbs of sliced yellow onions and stir occasionally for 45-60 minutes until caramelized.

Once the onions are golden brown, add 1.5 tablespoons of thyme and 6 minced garlic cloves. Stir for about 2 minutes until fragrant.

Pour in 0.5 cup of dry white wine, scraping the skillet's bottom to deglaze. Increase heat to medium-high and reduce the wine by half.

Sprinkle in 2.5 tablespoons of all-purpose flour and stir for 1-2 minutes until thickened. Add 2.5 teaspoons of Dijon mustard and 1 teaspoon of Worcestershire sauce, mixing well.

Gradually pour in 2.5 cups of beef stock and add 1 large bay leaf. Bring to a gentle boil then reduce heat and let simmer for 6-7 minutes.

In a separate pot, boil water and cook 24 oz of gnocchi until they float, about 2-3 minutes. Drain and add to the skillet with onion sauce.

Preheat the broiler. When ready, top the gnocchi with 1.5 cups of grated Gruyère cheese and 0.5 cup of Parmesan cheese. Broil for 3-5 minutes until melted and golden.

Remove from oven, discard the bay leaf, and garnish with fresh thyme. Serve hot.