Heat oven to 350°F. Spray 13x9x2-inch (3-quart) baking dish with cooking spray. Place hash brown potatoes and 1 tablespoon of the taco seasoning mix in large bowl; toss to coat potatoes evenly. Pat in baking dish.
Spray 10-inch skillet with cooking spray. Cook breakfast sausage and onion over medium-high heat 5 to 7 minutes, stirring occasionally, until thoroughly cooked; drain.
Meanwhile, beat eggs, cheese, milk, salt, pepper and remaining taco seasoning mix in same bowl with whisk until well mixed. Stir in sausage mixture and salsa. Carefully pour over hash browns in baking dish.
Bake uncovered about 40 minutes or until eggs are set in center. Let stand 10 minutes before serving.