In a small saucepan, heat the beef stock to almost boiling. Remove from heat and set aside. Preheat the grill or broiler.
In a 12 to 14-inch saute pan, heat the butter and the pork fat over high heat until the butter foams and subsides. Add the onion and saute over high heat 3 minutes. Add the stock, peas, sugar, salt and pepper and saute 5 minutes.
Meanwhile, grill or broil the bread. Add the prosciutto to the saute pan, toss over high heat 1 minute, adjust the seasoning and serve with the grilled bread.
Recipe copyright 2000, Mario Batali. All Rights Reserved.