Preheat a skillet with 1 tablespoon butter (or oil) on medium-low.
Spread Dijon mustard on the inside of a split piece of baguette. Fill with the brie and apple slices.
Add sandwich to skillet, the press a heavy pan on top to weigh it down. Cook until golden and crisp, 3 to 4 minutes per side. Add more butter to the skillet, if needed, for second side.