Preheat oven to 350'F. Cook noodles in boiling water 20 minutes. Blend soup and milk into smooth sauce. Add onions and mushrooms. Drain noodles. Layer 1 1/2 cups noodles, 1/3 cup tuna and 1 cup broccoli in a 2-quart casserole. Pour in 1 cup of sauce. Repeat layers. Bake 20 minutes. Then add cheese and bake 15 more minutes. Makes 6 1-cup servings.