Drain the beans and mash them with a fork or put them in the food processor. You want them to remain a little chunky.
In a small pot, combine the mashed beans, green chiles, broth, juice of ½ lime, ½ teaspoon of salt, the garlic granules, and pepper. Set the heat to medium and cook for 8 minutes, then lower temperature to simmer and cover until ready to use.
Chop the tomatoes and place them in a colander in the sink to drain for 10 minutes. Dice the jalapeño and onion. Chop the cilantro, removing all large stems. In a large bowl, juice the remaining limes and combine with the tomatoes, jalapeño, onion, cilantro, and the remaining salt. Cover and chill the pico de Gallo until ready to use.
Slice the avocado and place the tostada shells in the oven to warm up for 3 minutes.
Layer the tostadas, starting with the bean mixture, then lettuce, cheese, avocado, and pico de Gallo.