Melt 2 tablespoons of unsalted butter over medium heat. Add 3 large sliced yellow onions and a pinch of salt. Cook for 20-25 minutes until golden brown and caramelized.

Pour in 4 cups of low-sodium beef broth and add a teaspoon of thyme. Increase heat slightly and let simmer uncovered for about 10 minutes.

Bring a large pot of salted water to a boil. Add 12 ounces of wide noodles and cook until al dente, usually around 8-10 minutes. Reserve 1 cup of pasta water before draining.

Stir the drained pasta into the pot with the broth and caramelized onions. Add reserved pasta water as needed for desired sauce consistency.

Preheat broiler. Transfer pasta mixture to an oven-safe dish and top with 2 cups of freshly shredded Gruyère cheese. Broil for 3-5 minutes until bubbly and golden brown.