Step #1 Bring broth to a simmer in a saucepan & keep at a bare simmer, covered.
Step #2 Cook onion in 2 tbsps butter in a 3- to 4-quart heavy saucepan over moderate heat, stirring every once in awhile, until softened, 3 to 4 mins.
Step #3 Add rice & cook, stirring, 1 min.
Step #4 Add wine & let simmer, stirring, until absorbed.
Step #5 Stir in 1 c simmering broth & cook at a strong simmer, stirring constantly, until broth is absorbed.
Step #6 Continue simmering risotto & adding broth, stirring constantly & letting each addition become absorbed before adding next, about 1/2 c at a time, until rice is just tender & creamy but still al dente, 18 to 20 mins (there will be leftover broth).
Step #7 Stir in peas, 2/3 c cheese, parsley, remaining 2 tbsps butter, zest, prosciutto, & salt & pepper to taste.
Step #8 If necessary, thin risotto with some of remaining broth.
Step #9 Serve as soon as possible, with remaining 1/3 c cheese.
Step #10 Serves 4 as an entree or 6-8 as a side dish.