Cook pasta according to package directions. While pasta is cooking, combine 1/3-cup peas, milk, flour, salt and pepper in a blender jar or food processor. Blend until very smooth. Add remaining peas (without blending) and set aside.
Heat oil in a large skillet over medium heat. Add onions and cook, tossing until onion is tender, about 5 minutes. Add milk mixture, reduce heat to low and simmer 3 minutes, stirring constantly.
When pasta is done, drain well, reserving ½ cup of pasta cooking liquid. Add sauce and reserved pasta liquid to pasta. Stir over low heat until pasta is coated with sauce. Transfer to a serving platter or plates. Sprinkle with salt, pepper and Parmesan cheese.