In a bowl combine the first 5 ingredients until you have a homogeneous mixture.
Roll out small meatballs and set aside.
In a large pot add the oil and sauté the onion over a medium flame until soft and translucent, about 5 minutes.
Add the stock and potatoes and bring to a boil. Add the carrots, celery and meatballs and bring to a rolling boil. Add the spinach and the orzo pasta. Cook until orzo pasta is al dente.