Brown the ground beef in a large skillet over medium heat, then stir in the taco seasoning until the meat is evenly coated.
Transfer the seasoned beef to the slow cooker and add the pinto beans, frozen corn, and mild salsa. Stir to combine.
Mix in 1 1/2 cups of the cheddar cheese and all of the Monterey Jack cheese until everything is evenly distributed.
Sprinkle the remaining 1/2 cup of cheddar cheese over the top in an even layer.
Cover and cook on low for 4 hours until the mixture is hot and the cheese on top has melted.
Sprinkle the sliced green onions over the casserole just before serving.