Start by roasting the veggies. Preheat the oven to 425°F. When the roasted vegetables are done, slice or chop according to preference.
Heat oil, and add the onion and garlic. Sauté until tender. Add the beans to the pot, mashing with a potato masher until slightly chunky.
Assemble the tostadas by spreading a generous portion the black bean mixture onto each tostada shell. Then add another layer of the roasted veggies.
Top with salsa, avocado, and a sprinkle of crumbled cheese. Finish off with a drizzle of Mexican crema, pickled onions, and a squeeze of lime juice.