Add the black beans, onion, pepper, garlic cloves, chili powder, paprika, cumin, cayenne, salt and pepper to a saucepan over medium-low heat. Add the stock and lime juice and stir. Simmer the beans uncovered, stirred occasionally, for 10 to 15 minutes, until the mixture starts to reduce and thicken. Set it aside to cool slightly. Remove the garlic cloves from the beans.
To make the tostadas, you can pan-fry them or crisp them up in the oven.
To pan fry, heat a skillet over medium heat and add the oil. Add the tortilla and cook for 1 to 2 minutes, then flip and cook for another 1 to 2 minutes. Place the tortilla on a paper towel to drain any excess grease, sprinkle it with salt and repeat with remaining tortillas.
To toast in the oven, preheat the oven to 425 degrees Place the tortillas on a baking sheet and brush both sides with the oil or use an olive oil spray on both sides. Sprinkle the tortillas with salt. Bake for 5 to 6 minutes, then flip and bake for 5 to 6 minutes more.
To serve the tostadas, add a few spoonfuls of the black bean mixture right on top of the crispy tortilla. Top with the micro greens or lettuce, then add on the avocado, tomatoes, pickled onion, grilled corn, green onions, cotija cheese and cilantro. Spritz with lime juice and serve immediately!