Heat a large skillet over medium. Add the olive oil and once it starts to shimmer, add the diced yellow onion along with a pinch of salt. Cook, stirring every 1-2 minutes, until the onion softens and begins to turn golden, about 5-7 minutes. Add the garlic and jalapeno and cook for 1-2 minutes, stirring regularly. Sir in the dried oregano, smoked paprika, and chili powder. Cook for 1 minute, stirring regularly, then add the entire contents of the can of black beans (including the liquid). Reduce the heat to medium low. Allow the beans to cook 3-4 minutes until warmed through. As the beans cook, roughly mash them to help thicken the mixture.

While the beans cook, make the guacamole. Add the flesh of both avocados to a small bowl and roughly mash them. Stir in the chopped cilantro, finely diced red onion, and diced mango. Add the juice from half of one of the limes and season with a pinch or two of salt. Mix, then taste and add more lime or salt to suit your tastes. 

Warm tostada shells if desired. Divide the black bean mixture between the 6 tostadas. Top with the mango guacamole and your desired toppings. Enjoy immediately!

I recommend only making as many tostadas as you are going to eat at a time as the longer they sit with the toppings, the tostada shells will absorb the moisture and become soggy. Store leftover black beans, guacamole, and tostada shells separately. The black beans and guacamole can be stored in the fridge in airtight containers. The black beans will keep up to a week but the guacamole is best if enjoyed within 1-2 days. Re-warm the beans in the microwave or on the stove with a splash of water.