Peel and halve the onion and cut into fine strips. Clean, wash, dry grate and halve the cherry tomatoes. Clean, wash, dry grate and cut radishes into thin slices. Wash chives, shake dry and cut into diagonal rolls. Mix cherry tomatoes, radishes and onion in a bowl, season with salt, pepper and sugar
Spread each slice of bread with half of the top layer. Spread half of the tomato mixture on each half of the bread. Sprinkle with chives