Grilled Quesadillas

4 tortillas
1 cup Monterey jack cheese, shredded
1 cup stemmed, coarsely chopped watercress or arugula
1 tablespoon thinly sliced red onion
1 tablespoon tomato, finely chopped
1 tablespoon garlic (optional)
1 serrano or jalapeņo pepper, finely chopped
Salt and pepper to taste

Lightly oil the grill rack and place it about 4 inches above the coals.

Over medium heat, grill the tortillas for about 1 minute. Turn over and sprinkle with the remaining ingredients. Cover loosely with heavy-duty foil (or the grill hood) and cook until cheese melts (about 1 minute), checking to ensure that tortillas do not burn.