Heat the oven to 350°F and lightly grease a 9 x 13-inch baking dish.

Brown the ground beef and diced onion in a skillet over medium-high heat for 8 to 10 minutes, breaking the beef into small pieces. Drain excess fat, then stir in the taco seasoning and about 2/3 cup water, or the amount listed on the seasoning packet, and simmer for 2 to 3 minutes.

In a medium bowl, combine the refried beans, condensed cream of mushroom soup, and undrained Rotel tomatoes. Stir until the mixture is reasonably smooth and evenly combined.

Lightly crush the Doritos into coarse pieces rather than fine crumbs. Spread about half of the chips across the bottom of the prepared baking dish.

Layer the seasoned beef over the first layer of Doritos, spreading it as evenly as possible. Spoon the bean-and-soup mixture over the beef and gently smooth the surface.

Scatter the remaining Doritos over the bean layer, then cover the casserole evenly with the 2 cups of shredded cheese. Bake uncovered at 350°F for 25 to 30 minutes, until hot and bubbling and the cheese is melted.

Let the casserole rest for 5 to 10 minutes before serving. Add sour cream, green onions, tomatoes, black olives, jalapeños, or salsa just before bringing it to the table.