Melt a little butter in a skillet over medium heat. Place 1 of 2 slices of whole grain bread topped with nondairy mozzarella cheese in skillet. Grill until lightly browned and cheese is melted. Remove to plate. Melt some more butter and add the other slice of bread topped with cheese.
While second slice is grilling spread first slice with Celery Pesto and top with thinly sliced apple. When 2nd slice is done spread it with more Celery Pesto and place on first slice of bread.