Preheat the oven to 375°F and grease a 9x13-inch casserole dish.
Over medium heat, brown the ground beef and onion in a large skillet until beef is no longer pink. Drain excess fat.
Stir in taco seasoning and water. Simmer for 5 minutes or until thickened. Set aside.
In a large bowl, combine the thawed hash browns, sour cream, milk, salt, and pepper. Mix until well combined.
Spread the hash brown mixture evenly in the prepared casserole dish.
Layer the beef mixture over the hash browns.
In another bowl, mix the cream of chicken soup with the cheddar cheese soup. Spread over the beef layer.
Evenly spread the black beans, corn, and Rotel over the soup mixture.
Top with the shredded cheddar and Monterey Jack cheeses.
Bake for 30-35 minutes or until bubbly and the cheese is golden brown.
Let cool slightly before serving.