In the bowl of a food processor, combine the bell pepper with half the sliced onions and 4 of the garlic cloves; pulse just until the vegetables are finely chopped but not puréed.

Transfer the vegetables to a large bowl and add the lamb, beef, cumin, crushed red pepper, 2 teaspoons salt and 1 teaspoon pepper. Mix the ingredients with your hands or a flexible spatula until thoroughly combined. (The vegetables will be watery after they are chopped in a food processor, so you want to make sure the meat mixture is combined very well, until it’s almost sticking to the bowl as you mix.)

Line a rimmed baking sheet with parchment paper and grease the parchment with 2 tablespoons olive oil. Using lightly oiled hands, form about 24 small meatballs (roughly 1 heaping tablespoon each) and place on the oiled parchment, spaced about 1 inch apart. Transfer the meatballs to the oven and bake until they are golden brown and cooked through, 25 to 30 minutes.

While the meatballs bake, in a small serving bowl, whisk together the yogurt and 1 tablespoon lemon juice. Grate in the remaining garlic clove, season with salt and set aside for serving.

Soak the remaining sliced onion in a medium bowl of cold water for 10 minutes. Drain, dry thoroughly with paper towels and transfer to a large bowl. Add the parsley, mint, sumac, remaining 2 tablespoons olive oil and remaining tablespoon of lemon juice, and season with salt. Toss to combine and transfer to a serving bowl.

Serve the meatballs with the salad and garlic yogurt.