Cut tomato and onion into 8 (1/4-inch-thick) slices each. Layer in a shallow dish. Cover; chill 8 hours. Discard onion.
Stir together mayonnaise and mustard; spread on 1 side of bread slices. Place 1 tomato slice on each of 8 bread slices; sprinkle lightly with salt and pepper. Top with remaining bread slices. Cover and chill up to 2 days.