Toast 4 slices whole wheat bread. While the bread is toasting, cut 1 apple into thin slices and mix the apple slices with 2 tablespoons lemon juice to keep them from browning.
When the toast is ready, spread each slice with equal amounts of the 4 tablespoons mayonnaise and layer the apple slices on each piece.
Sprinkle ½ cup sunflower seeds evenly on top of the apple slices and top each sandwich with one each of 4 slices sharp cheddar cheese.
Toast until cheese is melted. Cut into triangles and serve immediately.