In a small bowl, dissolve yeast in milk. Let stand for 5 minutes. Add the flours, xanthan gum, oil, vinegar, sugar and seasonings. Mix until a ball forms (dough will be soft).
Sprinkle cornmeal over a 7-1/2-in. pizza pan coated with cooking spray. Press dough onto prepared pan, building up edges slightly. Do not let rise. Bake at 425° for 8-10 minutes or until lightly browned.
Spread pizza sauce over crust. Top with onion, green pepper, mushrooms and cheese. Bake for 8-12 minutes or until crust is golden and cheese is melted. Yield: 2 servings.