Remove the stem from the tomatoes, cut in half and finely puree in a blender. Remove the stem, the light-colored partitions and the seeds from the bell pepper and cut into small pieces.
Put the water and the tomato puree in a saucepan and bring to the boil, add the lentils and the pepper pieces with a little salt and sugar, to taste. Bring to the boil again and cook until firm to the bite for about 8-10 minutes.
Season the soup again to taste. If you like it spicier, you can add some chilli or pepper.
Put the soup in a bowl or on a plate and garnish with the finely chopped basil leaves.