Cook your pasta according to packet instructions. When the pasta is cooked, drain and set aside. Reserve about 1 cup of pasta water.
Meanwhile, heat oil in a medium sized saucepan. Once hot, add the brown onion and cook until the onions are soft and lightly browned.
Add the minced garlic and saute for 1 – 2 minutes, be careful not to burn your garlic.
Next, add the butter and melt. Once melted, add the grated Parmesan cheese and red pepper flakes (if using)
Turn off heat and season with salt and pepper to taste. You want to slightly over season your sauce.
Then mix through the roughly chopped parsley and cooked pasta. Once mixed through, if you want your pasta to have a bit more sauce, add some of the reserved pasta water and adjust seasoning as desired.