Roast the tomato paste in the olive oil, and add the celeriac and the whole onion that have been cut into larger pieces, then pour the bouillon onto. When it is boiled together, add the canned tomatoes (in the summer you can use softer tomatoes for preserving). Season it with salt and pepper. Stew it for about 1 hour in order to let the taste of vegetables cook into the stock. (When making a smaller proportion the time of cooking is also reduced.) When it is cooked, remove the vegetables and smash the soup.