In a bowl, combine ground beef, rice, grated onion, egg, salt, and black pepper. Mix gently until combined.
Shape into small meatballs, about hazelnut or large marble size.
Bring water to a boil in a pot. Add salt and avocado oil.
Add the meatballs and cook until the rice inside softens and the meatballs are fully cooked.
In a bowl, whisk lemon juice, flour, and a pinch of salt until smooth.
Whisk a ladle of hot cooking liquid into the lemon mixture, then slowly pour it into the pot while stirring continuously. This helps the sauce blend smoothly without curdling or forming lumps.