Preheat oven to 375°F. Grease a 9x13 baking dish.
Brown the ground beef in a skillet over medium-high heat until cooked through, 5-7 minutes, making sure to crumble the beef as it cooks. Add the diced onions and cook for 2-3 more minutes until translucent.
Stir in the taco seasoning and water. Allow to simmer for 2-3 minutes until slightly thickened.
Remove from heat and stir in the tomatillo salsa, sour cream, black beans and corn until well combined.
Crush half of the corn chips and spread them into an even layer in the prepared baking dish. Pour the beef and bean mixture evenly over top.
Sprinkle the shredded cheese in an even layer over top. Top with the remaining uncrushed corn chips.
Cover with foil and bake for 15 minutes. Remove foil and bake for 5 more minutes until lightly browned on top.
Allow to cool slightly before serving with desired toppings like chopped cilantro, avocado, lettuce, tomatoes, jalapeños and extra salsa!