Preheat your oven to 350°F (175°C). Lightly grease a 9-inch square pan or a 2-quart baking dish with cooking spray.
Warm the olive oil in a large skillet over medium-high heat. Add the ground beef and cook, stirring occasionally and breaking up the meat, for about 5-6 minutes until browned.
Add the diced onion to the skillet and continue cooking for an additional 3-4 minutes, or until the onion becomes translucent.
Mix in the taco seasoning and diced tomatoes (along with their juice). Allow the mixture to simmer for 3-4 minutes.
Arrange a layer of crushed tortilla chips evenly across the bottom of the prepared baking dish. Spread the refried beans over the chips in an even layer.
Spoon the seasoned beef mixture over the layer of beans, then sprinkle the shredded cheddar cheese on top.
Transfer to the oven and bake for 15-20 minutes, or until the cheese is fully melted and bubbly.
Garnish with your choice of toppings, such as shredded lettuce, diced tomatoes, and sliced olives. Serve immediately.