Cut asparagus into 2-inch lengths, steam briefly, drain, and set aside. Poach scallops in lightly salted water and keep warm with asparagus. Meanwhile, heat broth. Saute onion in butter; add rice to onion and stir over low heat for about 3 minutes. Add hot broth, a half cup at a time, stirring until liquid is absorbed before adding more. Continue until rice is tender, about 20 minutes. Remove from heat; add cream, Parmesan, scallops, and asparagus; and season to taste.