Position a rack in the center of the oven and heat to 425°F. Line a large rimmed sheet pan with parchment paper or foil. This is the timing enabler — a hot oven and a single pan keep everything at 30 minutes.

In a large bowl, combine the ground turkey, breadcrumbs, Parmesan, egg, garlic, Italian seasoning, 1/2 teaspoon of the salt, 1/4 teaspoon of the pepper, and the onion powder. Mix gently with your hands just until combined — overmixing makes dense meatballs.

Roll into about 16 meatballs, roughly 1 1/2 inches each (a heaping tablespoon per meatball). Arrange them on one side of the pan with a little space between them. In the same bowl, toss the bell peppers, zucchini, and red onion with the olive oil and the remaining 1/4 teaspoon salt and 1/4 teaspoon pepper. Spread the vegetables on the other side of the pan in a single layer.

Roast until the meatballs are golden and cooked through — an instant-read thermometer inserted into the largest one should read 165°F — and the vegetables are tender with browned edges, 18–20 minutes.

For the marinara version: spoon the marinara over the meatballs, sprinkle with mozzarella, and broil on high for 1–2 minutes, watching closely, until the cheese melts and spots. Skip this step for a plainer roast.

Shower everything with Parmesan and fresh basil. Serve straight from the pan with crusty bread, over rice, or tucked into rolls as meatball subs.