Evenly spread butter onto 1 side of bread
slices. Place 4 bread slices, buttered side
down, on work surface. Spread each slice
with mustard, then layer with cheese, tomato
and onion. Place remaining 4 bread slices,
buttered side up, over fillings
Heat griddle or large nonstick skillet over medium heat. Cook sandwiches 6 to 8 minutes or until bread is golden brown and cheese melts, turning once and pressing down occasionally with large spatula. Cut sandwiches diagonally in half to serve.