Remove the tough, rubbery appendage from the side of each sea scallop or dice the tuna. Core, seed, and chop the pepper. Mince the garlic or shallot. Stem and rinse the watercress or spinach; chop the spinach if using but leave the watercress leaves whole.
Heat the oil in a large skillet over medium heat. Add the peppers, reduce the heat, and cook until somewha