SAUTE onions in dressing in large saucepan on medium heat until onion is tender and dressing is absorbed.
HEAT oil in large skillet. Add mushrooms; saute until tender. Set aside.
FOLD each sheet of phyllo dough in half lengthwise; fold in half again crosswise. Brush lightly with butter. Working your way around the phyllo, fold and crimp edge to form a 1/2-inch rim while working phyllo into free-form circular shape. Brush edge with butter.
FILL each phyllo tartlet with layers of 1/4 cup mushrooms, 1/4 cup onion and 1-1/2 tsp. cheese. Place on lined full size sheet pans.
BAKE at 350°F for 10 minutes or until filling is thoroughly heated and crust is golden brown.