Cook Beef:
In a large skillet, cook ground beef and diced onion over medium heat until beef is browned and onion is softened. Drain excess grease.
Add taco seasoning, water, and drained Rotel tomatoes. Simmer for 5 minutes until slightly thickened.

Assemble Casserole:
Preheat oven to 350°F (175°C).
In a greased 9×13-inch baking dish, layer half the crushed Doritos evenly on the bottom.
Spread the beef mixture evenly over the chips.
Drop spoonfuls of sour cream over the beef, then sprinkle half the shredded cheddar cheese.
Top with remaining crushed Doritos and finish with the rest of the cheddar cheese.

Bake:
Bake uncovered for 20–25 minutes until cheese is melted and bubbly.

Serve:
Let cool slightly, then garnish with optional toppings. Serve warm for a crunchy, cheesy, taco-flavored family favorite!