Combine the yogurt, olive oil, lemon zest and juice, rosemary, salt, and pepper in a large, non-reactive bowl.
Cut the lamb in 1 ½-inch cubes (you’ll have about 20) and add it to the yogurt, making sure it is covered with the marinade.
Cover the mixture with plastic wrap and refrigerate overnight or for up to two days.
Prepare a charcoal grill with hot coals. Spread the coals in one tight layer on the grill.
Cut the red onion in 8 pieces and separate each piece into three or four sections.
Loosely thread three or four pieces of lamb onto skewers alternately with sections of onion.
Sprinkle both sides of the lamb cubes with salt and pepper.
Place the skewers on the hot grill and cook for 10 to 15 minutes, turning two or three times, until the lamb is medium-rare.
Serve with salad, grilled pitas and tzatziki.
Place the yogurt in a medium bowl.
Grate the cucumber on a box grater and squeeze the grated cucumber with your hand to remove some of the liquid.
Add it to the yogurt along with the sour cream, lemon juice, vinegar, dill, garlic, salt and pepper and stir.
Serve with grilled pita wedges.
