"Shashleek" (skewered meat) may be pork, if you look to Russian (agricultural legacy) customs. Or it may be lamb, if you look to Central Asian (nomadic legacy) customs.
For the skewers: Take the leg of lamb, butcher it down to 1-inch chunks (easier to grill faster) with a minimum of fat.
Puree the onions in a food processor and transfer to a large bowl. Add the mint, rosemary, salt, and lamb cubes; toss to coat. Cover the bowl with plastic wrap, refrigerate and let marinate for 1 to 3 days. Do not use vinegar!
Remove the lamb from the marinade; discard the marinade and weigh the lamb cubes out in 7-ounce portions. Thread lamb on skewers. Skewers can be prepared ahead of time and refrigerated.
To cook, heat a charcoal grill to high. Dust the skewers with salt, and grill to order. Before serving squirt lemon juice over them.
For the tzatziki:
Tzatziki (pronounced "sod ZEE kee") is a traditional Greek side. Take thick Greek yogurt and mix in lots of minced garlic, shredded cucumber, and dill to create a chunky vegetable dish.
Can be adjusted to taste, with sprinkles of sugar (it cuts the sharpness of the garlic). The thickness can also be adjusted (but the body should be like a thick tomato sauce, but short of a stiff porridge).