Peel and finely dice the onion and garlic. Heat the oil in a pot. Sauté onion and garlic in it. Stir in the tomato paste, sweat it on and deglaze with tomatoes. Bring to the boil, puree finely and season to taste with salt, pepper and sugar.
Simmer covered for 3-4 minutes. Wash the basil, dab dry, pluck the leaves from the stalks and chop finely, except for something to garnish. Dice mozzarella and add to the soup with pesto. Arrange the soup in a sauce boat, sprinkle with basil and garnish with basil leaves.