Peel and finely chop the onion. Celery peel, wash and cut into small cubes. Clean, wash and cut leek into fine rings.
Heat the oil in a large pot. Fry the onion for 3-4 minutes until transparent. Add celery and leek, fry for about 2 minutes. Deglaze with about 1.2 l water. Stir in stock. Bring to the boil, cover and simmer for 8-10 minutes.
In the meantime, preheat the oven (electric cooker: 200 °C/circulating air: 180 °C/gas: see manufacturer). Cut bread slices in half lengthwise. Place bread strips on a baking tray lined with baking paper. Grate cheese and spread on bread strips.
Bake in a hot oven for 5-7 minutes.
Add frozen peas to the soup, bring to the boil again and simmer for about 3 minutes. Remove from the stove. Roughly puree the soup with a hand blender (whole peas should still be visible). Season to taste with salt and pepper.
Wash the chives, shake dry and cut into small rolls. Pour soup into bowls. Sprinkle with chives and spread with cheese breadsticks.