Slice a boneless chicken breast into thin strips about 1cm wide. Season with salt and pepper and sauté in butter in a frying pan. Remove the chicken when it starts to brown, after about one minute, and set aside to keep warm. (On a plate with a glass bowl on top will do.) Add a little more butter or a drop more oil to the pan, then cook a handful of sliced mushrooms until softened, about 2 minutes. Lift them out with a draining spoon and add to the chicken. Soften a few shredded lettuce or watercress leaves and stems in the butter left in the pan, lift them out and stir in a couple of tablespoons of mayonnaise. Spread two soft baps or crusty rolls, split and toasted, with the mayonnaise and then pile on the chicken, mushrooms and leaves.