Heat oven to 375 degrees. If using dough sheet, unroll dough on work surface; starting in center, press into 12-by-9-inch rectangle. If using crescent dough, unroll dough into 1 large rectangle on work surface. Press into 12-by-9-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 muffin cups.
If necessary, crumble larger pieces of cheese into finer crumbs. Spoon cheese evenly into dough-lined cups; top each with prosciutto.
In small bowl, beat egg, cream and onions until blended. Spoon slightly less than 1 tablespoon mixture into each cup; sprinkle with walnuts.
Bake 15-20 minutes or until edges are golden brown and filling is set. Cool 5 minutes. Remove from muffin cups.