In a large pot, bring salted water to a boil. Add the fettuccine and cook according to package instructions until al dente. Drain and set aside.

In a skillet, melt the butter over medium heat. Add the sliced onions and cook, stirring occasionally, for about 20 minutes until they are caramelized and golden brown.

Add the minced garlic to the skillet and cook for an additional 1-2 minutes until fragrant. Pour in the beef broth and add the dried thyme. Bring to a simmer.

Add the cooked pasta to the skillet and toss to combine. Cook for another 2-3 minutes until heated through. Season with salt and pepper to taste.

Serve the pasta in bowls, topped with shredded gruyere cheese. Optionally, broil briefly to melt the cheese.