Cook the quinoa: Rinse quinoa under cold water. Add to a pot with the vegetable broth.
Bring to a boil, reduce to low, cover, and cook 15 minutes. Remove from heat and let sit, covered, 5 minutes. Fluff with a fork.
Prep the peppers: Heat oven to 375°F (190°C).
Slice the tops off the bell peppers and remove seeds and membranes. If needed, trim a tiny slice off the bottom so they stand upright without cutting through.
Sauté the aromatics: In a large skillet, warm olive oil over medium heat. Add onion with a pinch of salt and cook 3–4 minutes until softened.
Stir in garlic and cook 30 seconds until fragrant.
Cook the veggies: Add diced zucchini and a pinch of pepper. Sauté 3–4 minutes until just tender. Stir in cherry tomatoes and cook 1–2 minutes to soften slightly.
Season the filling: Add oregano, cumin, lemon zest, and lemon juice.
Fold in olives, capers, chickpeas, and cooked quinoa. Taste and season with salt and pepper. Off the heat, stir in parsley and feta.
Stuff the peppers: Spoon the quinoa mixture into the peppers, gently packing it in.
Arrange peppers upright in a baking dish. Pour 1/4 cup water or broth into the bottom of the dish to create steam.
Bake: Cover loosely with foil and bake 20 minutes. Remove foil, sprinkle with a little extra feta if you like, and bake 10–15 more minutes until peppers are tender but not collapsing and tops are lightly golden.
Finish and serve: Let rest 5 minutes.
Garnish with extra parsley and a squeeze of lemon. Serve warm.