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