First, get your oven ready by preheating it to 350°F (175°C). While it heats up, lightly grease a 3-quart baking dish with nonstick cooking spray. This simple step ensures that your casserole won't stick, making serving and cleanup much easier later.
In a large skillet over medium heat, cook the ground beef until it's browned all the way through. Once cooked, drain off any excess grease. Return the skillet to the heat and add the chopped onion and red bell pepper, cooking for about 5 minutes until they become tender.
Next, add the taco seasoning packet and the water to the skillet. Let the mixture simmer for about 5 minutes, allowing it to thicken into a rich sauce. Stir in the drained diced tomatoes with green chiles, the rinsed black beans, and the sour cream until everything is well combined.
Now for the fun part: layering! Start by sprinkling about 1 ½ cups of the lightly crushed Doritos in an even layer on the bottom of your prepared baking dish. Then, spoon about half of the beef mixture over the chips, spreading it out evenly.
Top the meat layer with another 1 ½ cups of crushed Doritos and 1 ½ cups of the shredded Cheddar-Jack cheese. Repeat the process with the remaining meat mixture, followed by the rest of the crushed chips and finally, the remaining 1 ½ cups of cheese on top.
Place the assembled casserole into the preheated oven. Bake for about 25-30 minutes, or until the cheese is completely melted, bubbly, and slightly browned on top. The filling should be heated all the way through.