Black Bean Tostadas with Mango Slaw

For the mango slaw:
1 mango, peeled and diced
3 cups shredded red cabbage
1 red bell pepper, diced
Juice of 1-1/2 limes
1/4 cup chopped cilantro
1/2 tablespoon olive oil
1/2 teaspoon salt

For the black bean mixture:
2 tablespoons olive oil
1/2 cup yellow onion, diced
Jalapeno, diced (seeded if desired)
2 (15.5 ounce) cans Michigan Black Beans, drained and rinsed
1-1/2 teaspoons salt
1 teaspoon chili powder
1/2 teaspoon garlic powder
1/2 teaspoon cumin
1/2 teaspoon oregano
1/4 cup water
1/2 lime, juiced
8 store-bought tostada shells
Crumbled queso fresco, avocado, and sour cream for serving