Spread French bread slices with mustard. Place hot dog halves, cut side down, over mustard on bread slices. Place on ungreased cookie sheet. Broil 4 to 6 inches from heat 3 to 4 minutes or until hot.
Remove from broiler. Place half slice of cheese on each sandwich. Broil 1 to 2 minutes longer or until cheese is melted. Top with relish and serve.