For the bean mixture, chop the garlic and onion and sauté in a little oil. Also chop the pickled tomatoes. Add the beans with the liquid and the remaining bean ingredients and simmer until the bean water has almost boiled off. Season to taste with salt and pepper.
For the guacamole, chop the coriander and mix with the remaining ingredients. Season to taste with salt and pepper.
Cut the mushrooms into slices and fry until the water has boiled off. Season to taste with salt and pepper.
Cover the lower half of the tortillas with guacamole, mushrooms and bean paste. Close the upper half. Heat some oil in the pan and fry the quesadillas for 2-3 minutes on each side. Place a saucepan or a smaller pan on the quesadillas while they are frying to prevent them from opening.