Cook your pasta according to packet instructions. Once cooked, drain and set aside. Reserve about 1 cup of pasta water.
While the pasta is cooking, heat half the butter in a medium sized saucepan. Once hot, add the finely chopped 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.
Add the remaining butter and melt. Once melted, add the grated Parmesan cheese.
Turn off heat and mix through the lemon juice and lemon zest.
Season with salt and pepper to taste. You want to slightly over season your sauce.
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.