Preheat oven to 350°F (175°C).
In a large skillet, heat vegetable oil over medium heat.
Add diced onion and minced garlic to the skillet and sauté for 3-4 minutes until softened.
Add ground beef to the skillet and cook until browned, breaking it apart with a spoon, about 6-8 minutes.
Drain excess fat from the skillet.
Stir in taco seasoning mix and water, then add the drained diced tomatoes with green chilies.
Simmer the mixture for 5 minutes until slightly thickened, stirring occasionally.
In a large mixing bowl, combine sour cream and salsa until smooth.
Spread half of the crushed Doritos evenly in the bottom of a 9×13 inch baking dish.
Layer half of the seasoned beef mixture over the Doritos.
Spread half of the sour cream and salsa mixture over the beef layer.
Sprinkle 1 cup of shredded cheddar cheese evenly on top.
Repeat layers: remaining crushed Doritos, remaining beef mixture, remaining sour cream and salsa mixture, and remaining shredded cheddar cheese.
Cover the baking dish tightly with aluminum foil.
To freeze: Place the covered casserole in the freezer for up to 3 months.
To bake from frozen: Preheat oven to 350°F (175°C). Bake covered for 50 minutes, then uncover and bake an additional 10 minutes until cheese is melted and bubbly.
To bake from thawed: Thaw casserole overnight in the refrigerator, then bake uncovered at 350°F (175°C) for 25-30 minutes until heated through and cheese is melted.