Cheesy Dorito Casserole

Preheat oven to 350°F (175°C). Grease a 9x13-inch baking dish with olive oil or nonstick spray.
In a large skillet over medium heat, warm olive oil. Add chopped onions and ground beef. Cook until beef is browned and onions are soft, breaking it up with a spoon (about 7 minutes). Drain excess fat if needed.
Stir in taco seasoning, then add the diced tomatoes (with juices) and cream of mushroom soup. Mix well. Simmer for about 5 minutes, until slightly thickened. Add a splash of water if mixture becomes too thick.
Roughly crush the Doritos (about 2/3 of the bag) and scatter an even layer in the bottom of the prepared baking dish.
Spoon a third of the beef mixture over the Doritos, then sprinkle on chopped cheddar.
Repeat layers with more crushed Doritos, another third of the beef mixture, and more cheese. Continue layering, finishing with any remaining cheese on top.
Cover with foil. Bake for 20 minutes, then uncover and bake 15-20 minutes more, until the cheese is fully melted, bubbly, and golden.
Remove from the oven and let rest 5-10 minutes before serving for best texture and easiest serving.