1.Make meatballs. Combine ingredients in large bowl, season. Roll rounded tablespoons of mixture into balls.
2.Heat oil in deep frying pan, cook meatballs until browned. Remove from pan.
3.Cook onion in same pan, stirring, until softened. Return meatballs to pan with stock, bring to the boil. Reduce heat, simmer, covered, about 15 minutes or until meatballs are cooked through. Reserve ½ cup of hot cooking liquid. Stir spinach into pan, cook until wilted. Remove pan from heat.
4.Meanwhile, beat eggs with fork in medium bowl until combined, whisk in flour and juice. Whisk in reserved hot liquid, stir mixture into meatballs. Stir gently until mixture simmers, season to taste.