Combine soup, 3/4 cup milk, salt, chicken, vegetables (frozen), 1/2 of cheese and 1/2 can onions. Spread into casserole dish. Bake uncovered at 425 for 10 minutes. Combine biscuit mix, egg and 1/4 cup milk to form soft dough. Spoon over hot mix. Bake 20-25 minutes more until dough is brown and risen. Add therest of cheese and onions. Let cheese melt and serve.