Preheat oven to 350°F. Lightly grease a 9×13 baking dish with cooking spray. Alternatively, you can use a 10-inch cast iron skillet.
In a cast iron skillet or large skillet, add ground beef and onion and brown and crumble ground beef until no longer pink.
Drain, if needed, wipe skillet, and return to skillet. Add taco seasoning and stir well to combine. Set aside.
In a large bowl, add eggs and milk. Whisk to combine, then add salsa, salt and pepper, and ½ cup shredded cheese. Stir to combine.