In a Dutch oven set over medium-high heat, melt olive oil and butter together. Add sliced yellow onions, thyme, and bay leaves, seasoning with salt and pepper. Cook for about 10 minutes until onions soften.

Lower heat to medium-low and cook onions for approximately 40 minutes until rich golden brown, stirring often. Add minced garlic and cook for an additional minute until fragrant.

Pour sherry wine into the onion mixture and let simmer briefly, scraping up bits. Add beef stock and bring to a simmer, cooking for another 10 minutes.

Preheat your oven to 350°F. Tear the French bread into bite-sized pieces and set aside.

In a mixing bowl, beat eggs and whisk in about 2 cups of the onion mixture. Add torn bread and Gruyère cheese, tossing gently to coat.

Grease a baking dish with olive oil and transfer the bread mixture into it. Pour remaining onion mixture on top, ensuring even distribution. Bake uncovered for 35 minutes.

Switch the oven to broil and sprinkle remaining Gruyère cheese on top. Broil for 1–2 minutes until golden and bubbly. Cool slightly before serving.