In a small mixing bowl, combine the yogurt, mint and radish. Season with salt and pepper to taste and set aside.
In another bowl, stir together the ground lamb, red onion and a pinch of salt. Shape into 4 long skinny loaves or “kefta.” In a large skillet over medium heat, heat the olive oil until hot. Add the kefta and brown all over, until no longer pink inside, about 12 minutes.
Place kefta on each piece of m'smen and garnish with the yogurt sauce and diced tomato. Roll into a cone shape and wrap with wax paper or tin foil. Serve immediately.