Heat a cast-iron skillet over medium-low heat. Spread both slices of bread with mustard and top one slice with Brie and pear. Top with remaining slice and press together gently. Generously brush both sides of sandwich with oil.
Place sandwich in skillet and cover with a piece of foil. Place a half-filled teakettle or a heavy skillet on top to weigh it down and cook until bread is golden and cheese begins to melt, 1 to 2 minutes per side. Serve warm.
Got leftover chicken or turkey? Tuck it in here for a heartier lunch.