In a small bowl, stir biscuit mix and milk just until moistened. Turn onto a floured surface; gently knead 6-8 times. Roll out to an 11-in. circle; transfer to a greased 12-in. pizza pan. Build up edges slightly.
Combine the cheese, onion and mayonnaise; spread over dough. Sprinkle with Italian seasoning.
Bake at 400° for 15-20 minutes or until golden brown. Cut into wedges; serve warm. Refrigerate leftovers.