Preheat oven to 350°F. Spray a 1-quart baking dish with cooking spray. In saucepan combine frozen veggies and boiling water. Bring to boil again, cover, simmer 5 min. Drain well.
In another pan, melt butter, blend in flour, cook 1 min. Gradually stir in milk. Simmer, stirring, until smooth and thickened. Add cheese and seasonings. Mix in veggies and onions. Spread into casserole dish.