In a bowl, mix all the ingredients for the meatballs. The mixture should be dense and able to hold its shape. Depending on how moist the ground meat is (or if it's been defrosted from frozen) and the size of the egg, you may need a little more or less of the breadcrumbs (or semolina), so add them gradually,
Form the meat mixture into balls the size of about 1 1/2 tablespoons and place them on a parchment lined tray or large plate. Let them rest at room temperature. It's important for the meatballs to be consistent in size so that they all cook at the same time. To help with this, you can use a mini cookie scoop or a tablespoon measure.
Chop the onions, carrots, celery, celeriac and bell pepper.
Add the olive oil to your pot and heat it over medium heat. Add the veggies and sauté for a couple of minutes over medium heat.
Add the tomato paste, herbs and spices and sauté for a few more minutes.
Add the stock and diced tomatoes and salt (if necessary, depending on your chicken stock) and boil for 5-10 minutes or until the veggies start to soften.
When the veggies start to soften, add in the meatballs and continue boiling the soup for another 10-15 minutes. To check if the meatballs are done, remove one from the pot and cut it in half. If the middle isn't pink anymore, they're done. Alternatively you can check the internal temperature - they're done when they reach 74℃ (165℉).
When your soup is done, remove from the heat and stir in the vinegar. Taste the soup and add more vinegar if desired.
Finely chop a handful of fresh parsley and put it on top of the soup. Serve with sour cream.