In a medium covered saucepan cook the unpeeled pearl onions (if using) in a small amount of boiling water about 10 minutes or until onions are just tender. Drain. Cool onions slightly; trim ends and remove skins. (Or, cook the frozen onions in a medium saucepan according to package directions; drain.)
In the same saucepan melt the margarine or butter over medium heat. Stir in the vinegar, brown sugar, and pepper. Cook and stir about 30 seconds or until combined. Stir in the onions and the prosciutto or ham.
Cook, uncovered, for 7 to 8 minutes more or until the onions are golden brown and slightly glazed, stirring occasionally. Makes 4 servings.