Bring a pot of water to a boil, salt it and cook pasta to al dente. Heads up: reserve 1 cup of starchy cooking liquid before draining.
Meanwhile, over medium heat in medium skillet, melt butter, add garlic and onions and sauté gently for 7-8 minutes. Add peas and heat though.
Drain pasta and add it back to the pot with reserved water, ricotta, parsley, Parm and onion-pea mixture. Toss 2 minutes to coat and combine flavors.