I had this about ten million times when in Barcelona. There is a sandwich stand in Plaza Sant Jaume (in Barri Gotic) called "Conesa." The first sign that this might be a good place to eat was the line twenty people-deep at any given time of day. I was lucky enough to be staying with a friend about 2 blocks away. My favorite sandwich was the "pollastre," I am going to try to reproduce it here.
2 chicken breasts
1 tablespoon alioli
2 large pieces lettuce
1 tablespoon butter
Rinse chicken breasts in cool running water and pat dry with paper towel. Using a meat mallet, pound breasts until thin between two pieces of wax paper. Salt and pepper chicken. Cover and refrigerate for later.
Wash onion and cut into long, thin strips. Melt 1 tablespoon butter in large saute over medium heat and add onions, stir so that onions are coated with butter. Turn heat down to medium-low and stir occasionally, until onions are lightly golden (about 20-25 minutes).
Heat up about 1 tablespoon olive oil in clean sauté pan over medium-high heat. Add chicken to pan and turn down to medium. Cook about 4 minutes on each side or until juices run clear (you can also cut into chicken, pink=undercooked, white=good to go). Remove chicken and cut into thin strips.
Wash lettuce and cut into thin strips. Place caramelized onions, lettuce strips and chicken on tomato bread. Drizzle alioli over chicken and top with another piece of bread. Grill on panini press for 5 minutes.