In an 11-inch skillet, heat 2 tablespoons of olive oil and 2 tablespoons of salted butter over medium heat until melted. Add 2 pounds of thinly sliced yellow onions. Stir occasionally, ensuring the onions are coated in the fat, and let them cook for about 45-60 minutes, adjusting heat as necessary to prevent burning. Look for the onions to turn a deep caramel brown.

During the final 5 minutes of caramelization, stir in 1.5 tablespoons of fresh thyme and 6 minced garlic cloves. Continue to cook until the garlic is fragrant and the thyme is wilted, which should take around 1-2 minutes.

Pour in 0.5 cup of dry white wine to deglaze the skillet, scraping up any flavorful bits stuck to the bottom. Allow the mixture to reduce for about 3-4 minutes until slightly thickened.

Sprinkle in 2.5 tablespoons of all-purpose flour while stirring to combine thoroughly. Cook for another minute to eliminate the raw flour taste. Incorporate 2.5 cups of low sodium beef stock along with 2.5 teaspoons of Dijon mustard and 1 teaspoon of Worcestershire sauce. Drop in a bay leaf and bring the mixture to a boil.

After boiling, reduce the heat and let the sauce simmer for about 6-7 minutes, allowing it to thicken slightly. Stir occasionally to prevent sticking, and taste for seasoning.

In a medium pot, bring salted water to a rolling boil. Add 24 ounces of gnocchi and cook until they float to the surface. Once floating, drain the gnocchi and stir them into the skillet with the sauce.

Layer with 1.5 cups of freshly grated Gruyère cheese and 0.5 cup of grated Parmesan cheese. Mix gently, allowing the cheese to melt slightly into the hot sauce.

Transfer the skillet to a preheated broiler and broil for 3-5 minutes until the cheese is melted, bubbly, and golden brown.

Remove the skillet from the broiler, discard the bay leaf and garnish with additional fresh thyme if desired. Serve immediately.