1Heat oven to 350°. Spray rectangular baking dish, 13x9x2 inches, with cooking spray. Pat hash browns in bottom of baking dish.
2Melt butter in 10-inch skillet over medium heat. Cook onions and mushrooms in butter 5 to 7 minutes, stirring occasionally, until mushrooms are tender. Stir in mixed vegetables.
3Beat eggs, cheese, milk, salt and pepper in large bowl with wire whisk until well mixed. Stir in mushroom mixture. Carefully pour into baking dish.
4Bake uncovered about 40 minutes or until eggs are set in center.