Spread the dijon mustard on the brioche bread, place a slice of swiss cheese on each slice of bread. Place the brie cheese and mozzarella on one side of the sandwich and sandwich the two slices together. Cut into a diagonal half.
Place each half into the batter and coat well. Carefully place them into the oil and cook for 2-3 minutes on each side. Remove from the pan onto paper towels to soak up the excess oil. Cut each half in half again and dust with powdered sugar. Serve along side black berry preserves.