Heat oil and butter in large nonstick pot. Add onion; cook, stirring, until softened, 5 minutes. Add rice and squash; cook 5 minutes.
Add 1 cup of the hot broth. Cook, stirring, until broth has been absorbed, 1 to 2 minutes. Stir in wine; cook, stirring occasionally, until liquid is absorbed. Repeat with remaining broth, 1 cup at a time. After about 20 minutes, rice should be firm, yet tender, with a creamy sauce.
Stir in cheese, parsley, rosemary, pepper and prosciutto. Serve at once.