Peel and finely chop the onion. Knead the mince, diced onion and egg. Season with salt and pepper. Shape into small balls. Defrost the beans. Clean and wash the Romanesco and divide into small "turrets".
Cook in boiling salted water for about 5 minutes, drain. Clean, wash and cut the peppers into pieces. Heat the oil in a large pan. Fry the meatballs for about 3 minutes. Add the bell peppers, Romanesco and beans and fry for another 5-8 minutes.
Deglaze with broth and bring to the boil. Stir in cream and mustard, bring to the boil again. Season to taste with salt and pepper. Serve garnished with parsley.