Heat the olive oil and butter in a large oven-safe skillet over medium heat. Once the butter melts, add the sliced onions and a small pinch of salt.

Cook the onions for 45–60 minutes, stirring every few minutes. Allow them to slowly soften and caramelize rather than browning quickly. If the skillet becomes dry, add about 1½ teaspoons of water at a time and stir. About 40 minutes into caramelizing the onions, bring a medium pot of salted water to a boil.

When the onions are deeply golden brown and caramelized, stir in the thyme and garlic. Cook for about 2 minutes, until fragrant.

Pour in the ½ cup beef broth. Scrape up the flavorful browned bits from the skillet and cook for 3–4 minutes, or until the liquid has reduced by approximately half.

Stir in the flour and cook for 1–2 minutes. Add the Dijon mustard and Worcestershire sauce, stirring until everything is evenly incorporated.

Pour in the beef stock and add the bay leaf. Bring to a gentle boil, then reduce the heat and simmer for 6–7 minutes, or until the French onion sauce begins to thicken. Season with salt and freshly cracked black pepper to taste.

Meanwhile, add the gnocchi to the boiling salted water. Cook according to the package directions, or until the gnocchi float to the surface. Drain thoroughly.

Remove the bay leaf and transfer the cooked gnocchi to the onion sauce. Gently stir until every piece is generously coated.

Sprinkle Parmesan evenly over the French Onion Gnocchi, followed by the grated Gruyère.

Place the oven-safe skillet under the broiler for 3–5 minutes, watching carefully, until the cheese is melted, bubbling, and lightly golden.

Garnish with additional fresh thyme and serve immediately.