Quarter the tomatoes and remove the core, then cut the pulp into very small cubes. Peel the onion and garlic cloves and cut into small cubes. Put everything in a bowl and mix with salt, pepper, vinegar, sugar, chilli flakes and the grated lemon zest. Finally add the oil.
Now cut the bread open and rub with garlic, grill in the oven at 200 degrees for 5 minutes at top heat. Then fold the chopped basil into the tomato mixture. Spread the mixture on the bread. Arrange on a platter and serve.