Caramelize the Onions:

In a large skillet over medium heat, melt the butter with the olive oil. Add the sliced onions and stir to coat. Cook, stirring occasionally, until the onions are soft and caramelized, about 30 minutes. If desired, add a teaspoon of sugar to help with the caramelization process.

Add Garlic and Deglaze:

Add the minced garlic and thyme to the onions, cooking for another 1-2 minutes until fragrant. Pour in the white wine (if using) to deglaze the pan, scraping up any browned bits from the bottom. Allow the wine to reduce by half.

Simmer with Broth:

Add the beef or vegetable broth to the skillet. Season with salt and pepper. Let the mixture simmer for about 10 minutes, allowing the flavors to meld together.

Cook the Pasta:

While the onions are caramelizing, bring a large pot of salted water to a boil. Cook the pasta according to package instructions until al dente. Drain, reserving a cup of pasta water for later.

Combine Pasta and Onion Mixture:

Add the cooked pasta to the skillet with the onion mixture, tossing well to combine. If the sauce seems too thick, add a bit of the reserved pasta water to reach your desired consistency.

Add Cheese and Bake (Optional):

Preheat the broiler. Transfer the pasta to a broiler-safe dish (if your skillet is not broiler-safe). Top with grated Gruyere cheese. Broil until the cheese is bubbly and golden brown, about 3-5 minutes.

Serve:

Garnish the french onion pasta with fresh parsley before serving. Enjoy hot!