HEAT oven to 400°F. Cook mushrooms and onions in dressing in skillet until tender. Cool.
BRUSH 1 phylllo sheet with butter; cover with remaining phyllo sheets, brushing each layer with butter before stacking. Cut 10-inch circle from stack; discard trimmings.
CUT cheese horizontally in half; place 1/2 in centre of phyllo stack. Top with 1/2 the mushroom mixture, remaining cheese and mushrooms. Fold phyllo around cheese, folding as necessary to completely enclose cheese; brush with remaining butter. Place on baking sheet or in pie plate.
BAKE 15 min. or until golden brown. Serve with crackers.