01 - Combine grated Gruyère and cheddar with Brie pieces and seasonings. Butter one side of each bread slice and spread Dijon on the opposite sides.
02 - Enclose cheese between the mustard-coated surfaces. Place in a nonstick skillet over low-to-medium heat, buttered surfaces outward.
03 - Cover briefly to help warm the center, then uncover and turn carefully once the base is golden. Continue until cheese melts and both sides are crisp.
04 - Rest briefly before cutting. If the filling is too large for the bread, serve surplus cheese separately rather than forcing the sandwich closed.