Preheat the oven to 400°F. Line a large sheet pan with parchment paper or lightly grease it.
Finely chop the parsley. Dice or grate the small onion as finely as possible.
If the onion releases a lot of liquid, gently press out the excess moisture using a clean kitchen towel or paper towel.
Add the ground beef, parsley, finely chopped onion, Lebanese 7 spice, cinnamon, sumac, salt, and black pepper to a large mixing bowl.
Mix gently with your hands until the ingredients are evenly combined. Avoid over-mixing, which can make the meatballs dense.
Divide the mixture into approximately 16 evenly sized meatballs and arrange them on the prepared sheet pan.
Arrange the tomato and onion wedges around the kafta meatballs. Drizzle the tomatoes and onions with olive oil. Sprinkle them with sumac, salt, and black pepper, then toss gently to coat.
Bake for 18 to 22 minutes, or until the kafta meatballs are browned and reach an internal temperature of 160°F.
For additional browning, broil the meatballs and vegetables for 1 to 2 minutes, watching them closely.
Serve the kafta meatballs with white rice and the roasted tomatoes and onions. Spoon the juices from the sheet pan over the rice and drizzle with tahini sauce, if desired.