Preheat the oven to 400 F.
Peel and cube the butternut squash into 1" pieces. Discard the peel and seeds.
Heat the oil over medium high heat in a large frying pan. Once sizzling, add the butternut squash and stir to coat with oil. Keep over medium high heat for about 5 minutes, moving the squash cubes frequently to prevent sticking.
Turn the heat down to medium, and add the taco seasoning. Stir well to coat the squash. If the squash is sticking in the pan, add a few tablespoons of water. Keep stirring occasionally and cook for 10 minutes or until the squash can be pierced with a fork. remove from heat and set aside.
When the oven is preheated, spray a cookie sheet with cooking spray. Place four tortilla shells on it, and spray the top of them. Bake for 5 minutes, then flip and bake for 5 more. Set aside when done.
Assemble your other tostada ingredients in an assembly line. Start with butternut squash filling first, then top with cheese so it melts into the warm squash. Pile on tomatoes, salsa, lettuce, sour cream, and top with cilantro.