Prepare the filling: Place all the filling ingredients in a bowl and squeeze together with your hands until smooth. Taking 1 level teaspoon of the mixture at a time, roll into balls and place on a tray. Set aside. 

Prepare the outer shell mixture: Pour ground rice (or Cream of Rice cereal) into a food processor and pulse with the meat and salt until a smooth “dough” is formed. 
Roll 1 level tablespoon of the dough into a ball, and place it onto a separate tray.  
Continue until mixture is finished. 

Form and stuff the shells: Press your thumb into the center of one shell ball, and turn dough while pinching it into a thin, hollow cup. Take one of the filling meatballs and stuff it into the cup, then pinch it closed. Lightly dampen your palms with cold water, and roll it back into a smooth ball without cracks or lumps. If you prefer, you can pinch the ends to create a torpedo shape. (If you have extra meatballs, save them to toss into the soup.) 

Prepare the broth and serve: Bring the stock to a boil in a large pot over high heat. Reduce the heat to medium-low and add the celery, scallions (or onions), garlic, and parsley and mix well. Simmer partially covered, for 10 minutes.  

Add the carrots, potatoes, salt, and pepper. Grind the dried mint between your palms directly over the pot. Mix and continue to simmer 15 minutes, partially covered.

Add the lemon (or lime) juice and mix. Place the whole squash (or zucchini) into the pot and continue to simmer, uncovered, over medium-low heat for 10 minutes.

Gently drop each meatball into the soup, cover, and simmer about 45 minutes. 

Serve the soup: Just before serving, remove the squash (or zucchini), slice it into 1-inch rounds, and return it to the soup. Serve hot in a large serving bowl or in individual bowls.