For the sauce

1. Combine all the ingredients for the sauce in your food processor, and blend until smooth. Adjust seasoning if needed, and consistency by adding either more mayo or lemon juice if needed.
2. Transfer the sauce to a bowl and cover with plastic wrap until ready to use. You can do this a day in advance.

For the meatballs

1. In a large bowl add the lamb, butter, olive oil, dried oregano, granulated onion, ground cumin, smoked paprika, ground ginger, sumac (if using), cinnamon, turmeric, ground black pepper, crushed red pepper flakes and kosher salt. Using two forks, gently mix everything together so that the fat and seasonings are well combined with the meat. Then add all of the remaining ingredients except for the balsamic vinegar. Gently toss to combine everything, then cover with plastic and place in your fridge so the flavors marry, about an hour. You can do this a day in advance.
2. After an hour at minimum, form the meatballs. They should be able to fit your cucumber slices, so try not to make them too big. Place the shaped meatballs on a foil-lined baking sheet and place them in the fridge or freezer so that the butter hardens up, about 15-20 minutes. Meanwhile, preheat your broiler and set the oven rack about 6 to 8 inches below the broiler. 
3. Once your broiler is super hot, place the meatballs in the oven. Let them cook for about 4-6 minutes (this will depend on the strength of your broiler). A visual clue is that they’re starting to sizzle and release their oils. After about 4 to 6 minutes, remove the meatballs from under the broiler and brush them with balsamic vinegar. Place them back under the broiler to continue roasting, about 2 to 3 minutes longer. You’ll know the meatballs are ready when they’re firm (but springy) to the touch. If they’re mushy, they’re not done — so continue broiling. When the meatballs are done, let them rest a couple of minutes, then drain them on paper towels to remove excess oil. Top your cucumber slices with a small dollop of the lemon-herb yogurt sauce, followed by a meatball, then garnish with the feta crumbles, red onion, fresh cilantro and dill. Use toothpicks to keep them together if needed. 
4. Enjoy.