Peel carrots and coarse grate. Divide into 2 equal parts. Mince chicken breast. Add one finely chopped onion and one part of the carrot to the mincemeat. Also, add oat flakes, salt, ground black pepper and fine-cut tarragon leaves (2 tablespoons). Knead mincemeat until smooth, cover with a cling film and refrigerate for 10-15 minutes. Meanwhile, finely chop the second onion and fry it to transparency in a vegetable oil. Then add the remaining carrot and fry for 2-3 minutes. Pour in passata, add salt and sugar. Cook for 5 minutes, stirring. The sauce should not be too thick. Water your hands and shape meatballs from the mincemeat about 3 cm in diameter and gently put them into boiling sauce. Cook covered for 25-30 minutes. Serve with a sour cream and sprinkled with fresh greens.