Wash the tomatoes, cut them into slices and season with salt and pepper. Wash the basil and spin dry - put some leaves aside, chop the others. Clean the spring onions, cut into slices and sprinkle over the tomatoes.
Mix the chopped basil with white wine vinegar, pepper, salt, sugar and mustard, then stir in the oil.
Spread the dressing over the tomatoes and serve garnished with the basil leaves.
Tip: You can also put sliced mozzarella with the tomatoes.