Mash the beans into a paste with a sprinkle of cumin and lemon juice for taste.
Lay 4 tostadas on the board, top 3 of them with some humus, then stack each side with a selection of the ingredients. The lettuce, avocado, cheese, beans, cheese, and tomato, there is no set rules, so individualize each one.
To finish, place one filled tostada, onto of the other, and top with the unfilled one to complete.