Preheat oven to 425 degrees F. Line a baking sheet with parchment paper.

Prepare meatballs. Combine all of the ingredients including ground beef, ground turkey, grated onion, panko, parsley, milk, egg, tomato paste, garlic, smoked paprika, cumin, salt, and pepper in a large bowl. Mix until well combined. Shape mixture into about 14 large meatballs. Place meatballs on prepared baking sheet.

Bake meatballs for 10 minutes. Reduce oven to 325 degrees F and continue baking for 15 minutes or until golden brown and the juices run clear. Remove from the oven and keep warm.

Meanwhile, make the spiced tomato sauce. Heat 2 tablespoons oil in a large saute pan over medium heat. Add garlic and saute for 30 seconds. Add oregano, parsley, red chili flakes, coriander, and fennel and saute for another 30 seconds, stirring frequently.

Add tomato paste and cook until caramelized, about 2 minutes, stirring frequently. Add 2 cups of water and whisk until well combined. Bring to a simmer and continue to cook for 10 minutes. Combine remaining 2 tablespoons of water with 1 tablespoon cornstarch until well combined. Pour slurry into sauce and stir to combine. Season with sugar, salt, and pepper and simmer until sauce is thickened. Remove from heat and stir in chopped cilantro.

Make sumac onion salad. Combine red onion with mint, parsley, juice of ½ lemon, and olive oil. Season with salt and pepper. Let sit for 5 minutes.

Pour spiced tomato sauce in the bottom of a serving plate and place meatballs on top. Top with sumac onion salad or serve salad on the side. Serve immediately.