Set a box grater over a clean kitchen towel & run the zucchini along the sides, grating it into fine pieces. Roll the towel up & squeeze as much of the excess liquid from the zucchini as possible. Transfer the grated zucchini to a large bowl.
To the large bowl with the grated zucchini, add the grated yellow onion, garlic, basil, grated parmesan, panko breadcrumbs, egg, olive oil, Italian seasoning, & crushed red pepper flakes (if using). Season with S&P as desired. Use your hands to mix well. Once the breadcrumb mixture is combined, add in the ground turkey or chicken. Using your hands, mix just until combined – take care to avoid over-mixing.
Divide the zucchini meatball mixture into 15-18 equal portions, then gently roll each into a meatball slightly larger than the size of a golf ball, rolling just until smooth – take care to avoid pressing the meatballs together too compactly. For quickness & ease, I like using a spring-loaded scoop. The mixture yields 15-18 1 1/2-inch meatballs. Arrange the meatballs on the prepared baking sheet.
Lightly spritz the zucchini meatballs with cooking spray for a golden finish, then transfer the baking sheet to the oven. Bake 18-22 minutes, until cooked through.