In a large Dutch oven, add the butter and water on medium to medium-low heat. Once it starts foaming, add the onions. Cook for 15 minutes, stirring often. Place the lid on top and transfer to the oven for 35 minutes.
In the meantime, bring a large pot of salted water to boil.
After your sauce has cooked for 30 minutes, add in the Calabrian chili paste and heavy cream and stir until well combined.
Now drop your pasta into the boiling salted water and cook just until al dente according to your box instructions. Reserve one cup of pasta water before draining the pasta. Add the pasta to your Dutch oven and stir. Add in some of the pasta water to your liking (the more you add the thinner your sauce will be). I prefer a thicker sauce, so I only added about 1/2 cup. Serve immediately and enjoy!