Heat a large Dutch oven or pot over medium heat and melt the butter. Add the onions and a big pinch of salt. Stir to coat with the melted butter, cover and cook for 10 minutes until soft and translucent. Uncover and continue to cook over medium-low to medium heat, stirring occasionally to keep the onions from sticking, for another 30 to 45 minutes until caramelized.

Add the mushrooms and garlic and stir to combine with the caramelized onions. Season with salt and pepper and add the thyme. Cook, stirring occasionally, until mushrooms are softened, about 5 minutes.

Deglaze the pot by adding the white wine and port and scraping up any browned bits on the bottom. Stir in the flour to create a smooth paste and cook for another minute.

Gradually add the broth, water, and Worcestershire sauce, stirring regularly to avoid any lumps, and raise the heat to high to bring to a boil. Taste and season with salt and pepper. You may want to over-salt the mixture because a lot of the salt will absorb into the pasta as it cooks, leaving the resulting sauce less seasoned.

Stir in the pasta, lower the heat to medium-low, cover and cook, stirring occasionally until the pasta is al dente and most of the liquid has absorbed, about 15 to 20 minutes (depending on the pasta shape). I find it can take longer than what the pasta packaging indicates because it’s cooking with less liquid over a lower temperature.

Taste and season with salt and pepper if needed. Stir in the cheese, allowing it to dissolve and melt into the sauce. It will help thicken it slightly. Serve immediately with more cheese and chopped chives, if desired.