Though it's a hard cheese, aged Gruyère melts nicely. You could also use Swiss cheese.
Preheat broiler to high.
Arrange bread on a foil-lined baking sheet. Coat bread with cooking spray. Broil 1 minute or until toasted. Remove pan from oven; turn bread over. Divide cheese evenly among bread slices; sprinkle with chives. Broil 1 minute or until cheese melts.