In a small saucepan, cover the tomatillos with water and bring to a boil over medium heat. Reduce the heat and simmer until the tomatillos turn from bright to olive green, about 7 minutes. Drain and let cool.
Meanwhile, in a food processor, puree the beans, chopped garlic and onion. In a deep skillet or Dutch oven, heat the oil over medium heat; add the bean puree and cook, stirring, until slightly thickened, about 8 minutes. Season with salt and set aside.
Clean the food processor bowl. Puree the halved garlic cloves with the tomatillos, chipotle chiles and adobo sauce until almost smooth; add water to thin if necessary. Season the salsa with salt.
Halve the avocados, scoop the flesh into a bowl and mash. Using a garlic press, crush the remaining garlic and add to the avocado. Season with salt.
Reheat the beans, then spread on the tostadas and top with the guacamole, lettuce and cheese. Serve with the tomatillo salsa.