Transfer the sliced red onions to a medium bowl and sprinkle with sumac, tossing gently to coat each slice. Pour in olive oil and lemon juice, season with salt, and scatter the parsley over the top. Toss until everything is evenly combined. Cover and chill for 30 minutes to allow the flavors to develop. Keep refrigerated for up to four days.
Heat the olive oil in a large, deep skillet over medium heat.
In a large bowl, combine the ground chicken, grated zucchini, breadcrumbs, parsley, egg, lemon zest, garlic powder, salt, pepper, and cumin. Mix just until combined. Shape into 18 to 20 meatballs, about 2 tablespoons each.
Add the meatballs to the skillet and cook, turning occasionally, until browned all over, 8 to 10 minutes. Transfer to a plate.
Add the yellow onion and diced zucchini to the skillet and cook, stirring occasionally, until softened, 5 to 7 minutes.
Stir in the pearl couscous, Dijon mustard, parsley, garlic powder, salt, and pepper. Cook, stirring constantly, until the couscous is lightly coated in the seasonings, about 1 minute.
Pour in the water or broth and bring to a gentle simmer. Return the meatballs to the skillet, nestling them into the couscous.
Cover, reduce the heat to low, and cook until the couscous is tender, the liquid has been absorbed, and the meatballs are cooked through, 25 to 30 minutes.
Remove from the heat and let stand, covered, for 5 minutes. Scatter with more parsley and finish with a squeeze of fresh lemon juice before serving.