Preheat the oven: Set to 400°F (200°C). Line a baking sheet with parchment for easy cleanup.
Prep the zucchini: Halve each zucchini lengthwise.
Use a spoon to scoop out the centers, leaving about 1/4 inch border. Chop the scooped flesh and set aside.
Season and roast: Place zucchini boats cut-side up on the sheet. Drizzle with 1 tablespoon olive oil, sprinkle with salt and pepper, and roast for 12–15 minutes until just tender.
Cook the quinoa: In a small pot, combine rinsed quinoa and 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.
Sauté aromatics: Heat remaining 1 tablespoon olive oil in a skillet over medium.
Add onion and a pinch of salt; cook 3–4 minutes until softened. Stir in garlic and the chopped zucchini flesh. Cook 3–4 minutes until most moisture cooks off.
Build the filling: In a large bowl, combine cooked quinoa, onion mixture, tomatoes, olives, feta, parsley, mint, lemon zest, lemon juice, oregano, cumin, and a pinch of red pepper flakes if using.
Season with salt and black pepper. Taste and adjust lemon or herbs as needed.
Stuff the boats: Spoon the quinoa mixture generously into each zucchini. Pack lightly so it holds together.
Bake to finish: Return stuffed zucchini to the oven for 8–10 minutes, just to meld flavors and warm through.
If you like a golden top, broil for 1–2 minutes at the end.
Serve: Top with a little extra feta, a squeeze of lemon, and a spoonful of Greek yogurt or tzatziki if you like. Add a few fresh herb leaves for color.