Dissolve milk powder, salt, mustard, pepper and corn starch in water, set aside. Saute onion in olive oil until tender. Stir in milk mixture, and bring slowly to a boil stirring constantly. Stir in cheeses and lemon juice, mix in cooked noodles. Put mixture into a greased 1 1/2 quart casserole. Bake 40-45 minutes in a 350 degree oven until hot and bubbly. ----