Remove core and cut apples into thin slices. Add apples to the skillet and cook for a few minutes each side until heated throughout and begininng to soften.
Spread mayonnaise on one side of the bread slices. Lower heat to medium, push apples to the side of the pan and place one slice of bread mayonnaise side down on the pan.
Place brie on the bread, top with the apples and the other slice of bread mayonnaise side up.
Toast each side until golden brown and the brie is beginning to melt. Slice and serve immediately.