Heat oven to 375°F. If using dough sheet: Unroll dough on work surface; starting in center, press into 12x9-inch rectangle. If using crescent dough: Unroll dough into 1 large rectangle. Press into 12x9-inch rectangle, firmly pressing perforations to seal.
Cut dough into 12 squares. Gently press squares into 12 ungreased mini muffin cups, shaping edges to form rims 1/4 inch above tops of cups. Spoon cheese evenly into dough-lined cups. Top with prosciutto.
In small bowl, beat egg, whipping cream and onions until blended. Spoon slightly less than 1 tablespoon mixture into each cup. Sprinkle with walnuts.
Bake 15 to 20 minutes or until edges are golden brown and filling is set. Cool 5 minutes. Remove from muffin cups. Serve warm.