For the sauce, melt butter in a saucepan which will be large enough to accommodate the pound of cooked pasta, and saute onion until it becomes translucent. Mix in the flour to make a roux. Gradually add milk and allow thickening over low heat. Remove from heat and whisk in Parmesan. Immediately cover surface of sauce with a sheet of plastic wrap and set aside briefly.
Boil the pasta until al dente and drain. Add the cooked pasta and the blanched peas to the pan containing the sauce and fold to coat pasta. Season with salt and pepper, as needed.