Boil the pasta according to the instructions on the package, but take it off the heat five minutes before the specified period; quickly dry the pasta and wash it with cold water so it stops being cooked.
Mix the minced meat with ¾ the amount of onions, a teaspoon of salt and another for black pepper. Mix well by hand and then make small balls of a size that fits in the shell’s cavity, and set them aside.
Mix the milk and garlic with a dash of salt and black pepper in a separate bowl, and leave the mixture aside.
In a skillet over medium heat, heat the oil, and fry the remaining amount of onions for 10 minutes until it turns golden; afterwards, add the tomatoes, a teaspoon of salt and another for the pepper; leave it over the heat for 10 minutes. Spread the mixture on the base of a medium-sized oven sheet.
Stuff each shell with a meatball and arrange in the sheet over the tomato mixture; repeat the process until the amount runs out.
Cover the sheet with tin foil, and place in the oven at 250oC for 15 minutes. Take out the sheet, and pour the yogurt-garlic mixture over it; garnish with fresh coriander and serve hot.