Mix the meat with the rice, salt, pepper and egg (and celeriac leaves, if desired). Sauté half of the onion in a little oil and then mix it into the meat. Stuff the meat mixture into the peppers, pushing the meat all the way into the peppers. Form meatballs with the remaining stuffing. In a pot, heat a little oil and cook the tomato paste. Add two to three litres of water, the celeriac leaves, the remaining half of the onion, salt and sugar. Bring to boil and add the stuffed peppers and meatballs. Simmer for 40 to 45 minutes. In a pan, heat 2 tablespoons of oil, and add flour. Stirring continously, prepare a light mixture, then stir it into the sauce and bring to a boil. Add salt and sugar to taste. Serve it with cooked potatoes.