HEAT oven to 375ºF.

COOK and stir mushrooms and onions in 2 Tbsp. dressing in skillet until vegetables are tender. Add water to stuffing mix in medium bowl; stir just until moistened. Stir in mushroom mixture and prosciutto; cool.

BUTTERFLY each turkey breast piece; pound to 1/2-inch thickness. Place, skin-sides down, on work surface; spread with stuffing mixture to within 1/2 inch of edges. Roll up, starting at one short end; tie closed with kitchen string. Place, seam-sides down, in 15x10x3/4-inch pan. Add squash, cut-sides up, to pan. Brush turkey and squash with remaining dressing.

BAKE 55 min. to 1 hour or until turkey is done (170ºF) and squash is tender. Cut and discard string before serving turkey with squash.