Preheat and Prepare: Position a rack in the center of the oven and preheat to 400°F (200°C). On a large rimmed sheet pan (18×13) or divided between two pans, combine potatoes, bell peppers, red onion, and chickpeas. Drizzle with 3 tbsp of the olive oil, season with 1/4 tsp of the salt and all of the black pepper, and toss to coat; spread in an even layer.
Par-Roast Vegetables: Roast for 20 minutes, stirring once halfway through.
Hydrate the Binder: Meanwhile, in a large bowl combine the whole-wheat panko, grated yellow onion, and minced garlic.
Form the Polpettes: Add the ground chicken, grated zucchini, egg, parsley and mint, lemon zest, dried oregano, smoked paprika, cumin, black pepper, and the 1/4 tsp kosher salt. Mix gently until just combined; form about 24 golf ball–sized meatballs.
Combine and Roast: Remove the pan; nestle the polpettes among the vegetables.
Make the Vinaigrette: In a small bowl, whisk together the remaining olive oil, lemon juice, Dijon mustard, oregano, and the remaining 1/4 tsp kosher salt until emulsified.
Final Bake: Drizzle about half of the vinaigrette over the polpettes and vegetables. Return to the oven and bake 20–25 minutes, until the polpettes reach 165°F (74°C). For extra browning, broil 1–2 minutes, watching carefully. Vegetables should be fork-tender with browned edges; if needed, roast 5–10 minutes more.