Split the rolls lengthways and pull out a little of the soft crumb from each half, leaving a shallow hollow. This makes room for the filling so the torta closes.
Warm the mashed beans in a small pan with a splash of water and half the salt until they are spreadable but not runny.
Toss the shredded chicken with the lime juice and the remaining salt.
Heat the oil in a skillet and toast the cut faces of the rolls, pressing them down, 1 to 2 minutes, until golden and dry to the touch.
Spread mayonnaise on the top half of each roll and the warm beans on the bottom half, right to the edges.
Layer the chicken onto the beans, then the queso fresco, tomato, onion, jalapeno and avocado.
Close each torta and press it down firmly with your palm, or weight it under a heavy pan for a minute.
Cut in half on the diagonal and serve at once, while the cut faces are still crisp.