Finely chop the garlic. Chop the onion finely. Pour yogurt into a bowl. Add onion and garlic. Season with salt, pepper, and stir. Set aside 2 teaspoons of the yogurt marinade sauce.
Dip the loin slices in the rest of the yogurt. Leave on for 3 hours.
Remove the loin from the yogurt.
Pour olive oil into a preheated grill pan. Put the loin slices, fry over high heat. Turn over and fry on the other side. Serve with the remaining sauce. Enjoy your meal!