Remove the avocado from their skins and mash in a medium-sized bowl.
Add the diced tomato, onion, cilantro and lime juice to the avocado bowl and stir through seasoning with salt.
Black Beans:
Heat a large frying pan over medium heat. Add the beans and spices and begin to mash them together as the beans heat up. Add water as needed until they have reached a spreadable consistency.
To Assemble: Spread some black bean mixture on a tostada, top with shredded lettuce, pico de gallo, guacamole and cheese then enjoy!