Bring broth to a simmer in a saucepan and keep at a bare simmer, covered.
Cook onion in 2 tablespoons butter in a 3- to 4-quart heavy saucepan over moderate heat, stirring occasionally, until softened, 3 to 4 minutes. Add rice and cook, stirring, 1 minute. Add wine and simmer, stirring, until absorbed.
Stir in 1 cup simmering broth and cook at a strong simmer, stirring constantly, until broth is absorbed. Continue simmering risotto and adding broth, about 1/2 cup at a time, stirring constantly and letting each addition become absorbed before adding next, until rice is just tender and creamy but still al dente, 18 to 20 minutes (there will be leftover broth).
Stir in peas, prosciutto, zest, 2/3 cup cheese, parsley, remaining 2 tablespoons butter, and salt and pepper to taste.
If necessary, thin risotto with some of remaining broth. Serve immediately, with remaining 1/3 cup cheese.