Peel and finely chop the onion. Melt the fat in a pot and sauté the onion cubes for about 2 minutes. Stir in flour and sweat briefly. Add 500 ml broth and milk while stirring. Bring to the boil, let it boil for about 5 minutes, season with salt and pepper. In the meantime peel and wash the celery and cut it into fine cubes. Bring 250 ml stock to the boil and cook the celery cubes for about 4 minutes
Heat the oil in a pan. Fry the ham in it until crispy. Wash the chives, shake dry and cut into small rolls. Grate cheese and sprinkle into the soup while stirring.
Add celery cubes with the stock to the soup. Season soup with salt and pepper. Break the ham into small pieces. Arrange the soup and sprinkle with pieces of ham and chives