In a large pot, melt the butter over medium heat. Add the sliced onions and a pinch of salt. Stir frequently for about 20-25 minutes until soft and golden brown.

Pour in the beef broth and add fresh thyme. Stir and bring to a gentle simmer for an additional 10 minutes.

In a separate pot, cook the pasta in salted boiling water according to package instructions until al dente, about 8-10 minutes. Reserve a cup of pasta water, drain the pasta, and set aside.

Combine the drained pasta with the onion and broth mixture, stirring until evenly coated. Adjust the sauce's thickness with reserved pasta water as needed.

Transfer to an oven-safe dish and sprinkle the shredded Gruyère cheese evenly on top. Broil for 3-5 minutes until the cheese is bubbly and golden.

Remove from the oven and let sit for a few minutes. Serve warm, perhaps with crusty bread.