In a large bowl, mix all the ingredients together and chill for up to 1 hour. Shape into 4 evenly sized burgers, brush with oil and grill, or fry in a nonstick pan over moderate heat, for 4-5 minutes each side, until cooked.
To serve, arrange 4 tortilla shells on individual serving plates, place a few salad leaves on each shell and top with a dollop of sweet po