Peel the lower third of the asparagus and cut off the ends. Cut the asparagus diagonally into 3 cm long pieces.
Quarter and core the tomatoes and cut into small cubes. Dice the mozzarella as well.
Pluck the basil from the stems, roughly chop 2/3 and put the rest aside.
Heat 1 tablespoon of oil in a pan and fry the asparagus in it, it should still have a bite. Dice the onion and fry briefly. Season the asparagus with salt, pepper and sugar and leave to cool.
Make a vinaigrette from vinegar, the remaining oil, salt and pepper and mix with all the salad ingredients.