Peel and chop the onion and garlic. Clean and wash the celery, put some green aside. Cut the celery stalks into slices. Pour beans into a sieve and rinse.
Heat 2 tablespoons of oil in a large saucepan, sauté the onion, garlic and celery for about 5 minutes. Add the beans and 1 1⁄4 l water. Stir in broth. Bring to the boil and cover for about 20 minutes.
Meanwhile, wash the lemon hot, dry it, and rub the peel thinly. Halve the fruit and squeeze out one half. Wash parsley, shake dry. Chop parsley and some celery green finely. Mix in lemon zest and 4 tbsp. oil.
Dice the bacon. Heat 2 tablespoons of oil in a frying pan and leave the bacon in it for about 5 minutes until crispy. Dice the bread, add it to the bacon in the pan and fry it until golden brown.
Finely puree the soup with a hand blender, season to taste with lemon juice, salt, pepper and 1⁄2 TL sugar. Serve, sprinkle with bacon croutons and sprinkle with some lemon parsley oil.