Oven Preparation: Begin by preheating the oven to a temperature of 375°F. Subsequently, ensure that a 2-quart baking dish is lightly coated with cooking spray.
Meat Preparation: In a pan, brown the ground beef on a medium flame until its color predominantly shifts from pink. Subsequently, introduce the finely chopped onions to the pan and sauté for a duration of 3-4 minutes or until the onions become translucent. Following this, incorporate the taco seasoning along with the water, ensuring a consistent mix. Allow this mixture to simmer for a few minutes until it achieves a slightly thickened consistency.
Sauce Incorporation: Following the above, introduce the tomatillo salsa and sour cream into the pan. Ensure a consistent stir. Sequentially add the black beans and corn, ensuring a uniform distribution within the mixture. Once done, remove the pan from the heat.
Casserole Assembly: Crush half of the Frito corn chips to a coarse consistency and evenly distribute them at the base of the previously prepared baking dish. This should form a complete layer. Once this base is set, evenly layer the previously prepared taco meat mixture over it. This should be followed by an even sprinkling of the Mexican-blend cheese. Conclude this layering by topping the casserole with the remaining corn chips.
Baking: Cover the baking dish and place it in the oven. Allow it to bake for a duration of 15-20 minutes or until noticeable bubbling occurs and the cheese achieves a fully melted state. Following this, uncover the casserole and allow it to bake for an additional 5 minutes. Once done, remove from the oven.
Final Presentation: Once out of the oven, and prior to serving, the casserole can be adorned with the optional garnishes as per individual preferences. It is recommended to serve this dish immediately, accompanied by additional salsa and sour cream, if desired.