Place a large pot over medium-high heat and add the butter. Once melted, add the onions, salt, and water. Bring to a boil and close the lid.
After 5 minutes, check the onions and stir. (If needed, turn the heat to medium so the onions don't burn.) Cook covered until the water is mostly evaporated, about 10-15 minutes.
Remove the lid and press down on the onions with a wooden spoon or rubber spatula. Keep cooking the onions, pressing down every 30 seconds until browned and caramelized, about 10 minutes. (Don't walk away from the stove!)
Deglaze the pot with dry sherry and white wine. Stir and cook for 1-2 minutes. Add the pasta, beef broth, Worcestershire sauce, thyme sprigs, and pepper. Stir to combine.
Cook for 12- 15 minutes. Use the pasta cooking time on the package as the reference, cooking longer if needed until al dente.
Remove the thyme sprigs and stir in the parmesan and heavy cream. Season with salt and pepper to taste.
Portion into bowls and garnish with more parmesan and some fresh thyme leaves, if desired.
Notes
You can substitute 2 teaspoons of dried thyme if you don't have fresh thyme on hand.
For a vegetarian option, swap out the beef broth for vegetable broth. To get more umami flavor, add 1-2 tablespoons of soy sauce when you add the broth.