Preheat the oven to 375°F. Cut the tops off the peppers and remove the seeds and ribs.
Heat the olive oil in a small skillet over medium heat. Cook the onion for 3–4 minutes, until soft. Add the garlic and spinach and cook for about 1 minute, until the spinach wilts. Remove from the heat.
Stir in the rice, chickpeas, tomatoes, oregano, salt, pepper and lemon juice. Fold in half of the feta and taste for seasoning.
Spoon the filling into the peppers, packing it in and mounding the tops.
Stand the peppers in a small baking dish. Add 2 tablespoons of water to the dish and cover tightly with foil.
Bake for 25–30 minutes, until the peppers are tender.
Uncover, sprinkle with the rest of the feta and bake for 5–8 minutes more, until the feta is lightly golden. Serve with a squeeze of lemon.