Preheat the oven to 200 ° top / bottom heat.
Finely chop the onion and mix with the cheese and the remaining ingredients. Spread the black bread slices on a baking tray lined with baking paper and brush with the cheese mixture.
Bake for 10-15 minutes, then switch to grill setting and grill for about 2 minutes (do not let your eyes out!) Until golden brown.