Melt the butter in a large pot or pan.
Slice up the 3 small white onions (or 2 large onions) and add them to the melted butter. Saute on medium to medium-high heat until the onions are starting to caramelize. This can take 10-15 minutes. Stir occasionally.
Add in seasonings while onions are cooking and stir.
When onions are starting to turn brown and carmelize, deglaze the pan by adding in one box of the beef stock and scrape the brown bits off the bottom of the pan. Stir and bring to a low boil, then add in the pasta.
Let the pasta boil in the sauce on medium to medium-high heat and gradually add in the second box of beef stock and 3 Tbsps of the Better Than Bouillon Premium Roasted Beef Base. Stir often to make sure the pasta isn't sticking to the pan.
Cook until the pasta is done. The stock should be just barely covering the pasta at this point. Add in the heavy whipping cream and 3/4 of the bag of grated Parmesan cheese. Stir. Taste the sauce at this point and if it needs more salt and pepper, add that in to taste if needed. The beef stock also has salt in it so don't over salt it until you've tasted it after it's all been added and simmered down some.
Let simmer for a few minutes until the parmesan cheese is melted into the sauce and it starts to thicken up a bit. Stir often to keep the pasta from sticking to the bottom of the pan. If the sauce is too runny, keep simmering on low, add more parmesan cheese, and stir into the sauce until melted. Turn off the heat and let it sit for a few minutes.
The sauce will continue to thicken as it sits. Serve up with grated parmesan cheese. (If for some reason you can't get the sauce to thicken, you can mix 2 tbsp of cornstarch with 1/4 cup water in a separate container and add that to the sauce. Let simmer to thicken up the sauce and that should get it a bit thicker) (If you have the opposite problem and the sauce is too thick, add in more beef stock to thin out the sauce.)