Preheat oven to 350℉. Spray a deep sided 13"x9" baking dish with non stick cooking spray. Heat a large skillet over medium high heat. Add the sausage, onions, and peppers. Season with a pinch of salt. Cook, breaking up sausage, until sausage is cooked through, about 7-10 minutes. Drain well.
In a large bowl, whisk the Greek yogurt and milk. Add the eggs, 1 teaspoon salt, 1 teaspoon black pepper, whisk vigorously to blend eggs.
Layer the casserole: Place the hashbrowns in the bottom of the prepared dish (if using), followed by half of the sausage mixture, 1 heaping cup of cheese, the salsa, and half of the eggs. Spread the tortilla pieces evenly on the casserole and press firmly. Add remaining sausage, eggs, and cheese.
Bake covered tightly with foil 35 minutes. Remove cover and bake an additional 20-30 minutes, or until the casserole is cooked through and egg in the center of casserole is set. Allow to cool 10-15 minutes before garnishing with green onion, and serve with your favorite toppings.