I usually start by cutting off the side cheeks of the mangos, leaving a middle sliver that holds the pit. Score each cheek with a knife but be sure not to pierce the skin, similar to scoring an avocado half. Press the cheek down on the cutting board and slice off the squared bits of mango. Alternatively you can scoop them out with a spoon.
For the middle sliver of the mango you can run a paring knife along the inside edge of the skin to quickly remove it. Then start on the outside and slice off any juicy chunks of mango. As you get closer to the pit it'll get harder to cut. Once that happens you've probably retrieved most of the usable mango and can discard the pit.
Finely chop 1/4 onion, 1-2 rinsed and de-stemmed jalapenos, and 6-8 sprigs of rinsed cilantro. I typically twist off and discard the bottom, thicker portion of the cilantro stems, but use the upper stems that hold the leaves together. For a milder batch you can start with a single jalapeno and go from there. I used 2 jalapenos and it had some kick.
Add the chopped mango, onion, jalapeno and cilantro to a mixing bowl along with the juice of 2 limes and a pinch of salt. Combine well and take a taste, adding more jalapeno or lime juice if you want.
Drain and rinse a can of black beans (or use home-cooked beans). Add 1/2 cup black beans to the mixing bowl and combine well, takign a final taste for seasoning.
I served it on warm, crispy tostada shells along with a dusting of chile powder, freshly chopped cilantro, and a final squeeze of lime. For a light dusting of chile powder, simply add 1/2 teaspoon chile powder to a small cheese grater and hold it over your tostadas, tapping lightly. I baked my own tostada shells for this batch, but you can use store-bought tostadas as well. Here's how to bake your own tostada shells. Store in an airtight container in the fridge where it will keep for a few days.