Preheat the oven to 375. Put a large frying pan on medium heat. To it, add the olive oil, ground beef, onion, squash, and taco seasoning. Cook until done.
Place the taco shells out on a baking sheet. Add some of the meat and bake in the oven for 6 minutes.
Remove from the oven, add some cheese, lettuce, tomato and sour cream to each taco, and serve.