Add the sausage and onion to a large skillet. Cook over medium heat, breaking up the sausage with a spatula, until no pink remains. Drain any excess fat from the skillet.

Grease a 5 to 6 quart slow cooker. Add half the frozen hash browns to the bottom of the skillet. Spread half of the sausage and onion mixture on top. Top with half the peppers, half the green chilies, and half the cheese. Repeat the layers with the remaining hash browns, sausage, peppers, chilies, and cheese. Sprinkle with cilantro.

In a large bowl, whisk together the eggs, milk, salt, pepper, and garlic powder. Pour the mixture into the crock pot.

Cook on low for 7-8 hours or until eggs are set and temperature registers 160 degrees in the center. There will likely be liquid around the edges of the casserole once it's done. Use a paper towel to sop up any extra liquid before serving. Serve with salsa or sriracha, if desired.