Peel the onion and cut into fine strips. Quarter the tomato, clean, seed and cut into cubes. Dice the mozzarella. Put cheese, tomato cubes and onion in a bowl. Add 1 teaspoon of oil and vinegar and season with salt and pepper.
Wash the rocket and dab dry. Cut bread slices in half and roast them crispy in a grill pan without fat. Brush with remaining olive oil and sprinkle with salt. Arrange the rocket and mozzarella-tomato salad on the farmhouse bread and garnish with basil