Step 1: In a large, heavy-bottomed pot or Dutch oven, melt the butter over medium-low heat. Add the thinly sliced onions and 1/2 teaspoon salt. Cook, stirring occasionally, for 15-20 minutes, or until the onions are deeply golden brown and softened.
Step 2: Add the minced garlic to the caramelized onions and cook for 1 minute until fragrant. Stir in the flour and cook for another minute, stirring constantly.
Step 3: Pour in the beef broth, scraping up any browned bits from the bottom of the pot with a wooden spoon. Bring the mixture to a simmer.
Step 4: Add the pasta to the pot, ensuring it’s mostly submerged. Cover the pot and reduce the heat to medium-low. Cook for 8-10 minutes, or according to package directions, stirring occasionally to prevent the pasta from sticking.
Step 5: Once the pasta is al dente and most of the liquid has been absorbed, stir in the heavy cream and half of the grated Gruyère cheese. Cook for another 2-3 minutes, stirring constantly, until the cheese has melted and the sauce is creamy and coats the pasta. Taste and adjust seasoning with salt and freshly ground black pepper as needed.
Step 6: Ladle the pasta into bowls. Top each serving with the remaining grated Gruyère cheese. Garnish with fresh chopped parsley or chives, if desired, and serve immediately.