Open the black beans can and drain, reserve on a cup of their liquid. Place the beans and the reserved liquid in your blender and process until you have a very creamy and smooth texture. Set aside.
In a medium-size skillet heat the one tablespoon of the vegetable oil, once hot pour stir in the onion and cook until transparent, about 2 minutes. Add the bean mixture, and the epazote leaves, if using. Cook for 5 minutes until warm stirring frequently. Season with salt and set aside. Do not let the beans mixture dry, once the beans are warm, remove the skillet from the heat.
To make the tostadas: Pour one cup of vegetable oil in a large frying pan. Heat over medium-high heat. Once the oil is hot, add the tortillas, frying one by one until they are crispy and golden brown, this step will take about 1-½ minutes. Place the already fried tortillas in a plate covered with paper towels to drain any excess oil. Set aside.
To assemble the tostadas, spread 2 tablespoons of the creamy black beans over each tostada, evenly top with the shredded lettuce, sprinkle with the crumbled queso fresco and drizzle with Mexican cream. Add your favorite salsa and enjoy immediately!