Heat oven to 450°F. Combine diced squash, 1 tablespoon olive oil, 1 teaspoon salt and 2 teaspoons of your favorite taco seasoning on a baking sheet and toss until squash is well coated. Spread squash out to a single layer and roast until squash is soft and starting to brown, about 15 to 20 minutes. Remove from oven and set aside. (This can be served warm or at room temperature.)

For the Beans: 

Heat 1 tablespoon olive oil in a small saucepan over medium heat. Add 1/2 diced onion and 1 clove minced garlic and season with salt. Cook, stirring occasionally, until onions are translucent. 

Add the can of black beans (juice and all), and 2 teaspoons taco seasoning. Bring to a boil, then reduce heat to a simmer. Using a potato masher, or the back of a large spoon, mash beans until almost all are broken up. Let simmer until thick, about 5 minutes. Taste and add more salt if necessary.

I like to place all the components on the table and let my family build their own tostadas. I also place bowls of lime wedges, avocado slices and bottles of hot sauce out for garnish. To keep them vegan but still add some richness I like to top them with chopped pistachios. You can also garnish the tostadas with crumbled queso fresco (or feta cheese), cilantro leaves and sliced radishes. 

If you want to serve the tostadas already made: take a tortilla and spread some beans on one side, a couple tablespoons full will do. Top with about 1/4 cup of the squash and a small mound of slaw.