Prep the shells : Heat oven to 425°F (220°C). Lightly oil a large high-sided roasting pan. Slice tops from peppers and tomatoes; scoop tomato pulp/juices into a bowl and reserve. Set hollow peppers and tomatoes upright in the pan; keep the tops for lids.
Par-cook grains and lentils : In a saucepan, cover lentils with cold water by 2 inches. Bring to a boil, then simmer 10 minutes. Add rinsed brown rice and ½–¾ cup reserved tomato liquid; simmer 8–10 minutes more until grains are about 70% cooked. Drain well (reserve any remaining tomato liquid).
Start the wedges : On a rimmed sheet pan, toss potato wedges with 1 tablespoon olive oil, ¼ teaspoon salt, ¼ teaspoon pepper, and a pinch of paprika. Roast 20 minutes at 425°F, flipping once halfway. They will finish with the gemista.
Build the sofrito : In a skillet over medium heat, warm 1 tablespoon olive oil. Sauté onion with half the garlic, ½ teaspoon oregano, and paprika until glossy, 4–5 minutes. Fold in grated zucchini; cook 1–2 minutes to drive off moisture. Transfer to a large bowl.
Make the filling : Add par-cooked grains and lentils, mashed chickpeas, parsley, mint, lemon zest, ½ teaspoon salt, ¼ teaspoon pepper, and 1 teaspoon olive oil to the bowl. Toss until evenly moistened; add a spoon or two of reserved tomato liquid only if dry.
Stuff and nestle : Loosely fill peppers and tomatoes to about 90% (the grains will expand slightly). Cap each with its reserved lid. Tuck the par-roasted potatoes between the stuffed vegetables in the roasting pan.
Sauce, steam, and char : Whisk crushed tomatoes, ½ cup water, remaining oregano, remaining garlic, and the remaining ¼ teaspoon each salt and pepper. Pour around (not over) the vegetables. Drizzle the last 1 teaspoon olive oil over the pan. Cover tightly with foil and bake 30–35 minutes at 400°F (205°C). Uncover, increase heat to 425°F (220°C), and roast 20–25 minutes more until peppers and tomatoes are soft with blistered spots and the potatoes are deep gold.
Rest and serve : Rest 10 minutes to reabsorb juices. Spoon pan juices over everything. Shower with herbs and serve with lemon wedges. Add optional feta at the table if desired.