Preheat the oven: Set to 375°F (190°C). Lightly oil a baking dish that fits the peppers snugly.

Prep the peppers: Slice off the tops and scoop out seeds and membranes. If needed, trim a thin slice from the bottom so they stand upright, without cutting through the base.

Sauté aromatics: Warm 1–2 tablespoons olive oil in a skillet over medium heat. Add onion with a pinch of salt and cook until translucent, 4–5 minutes. Stir in garlic and cook 30 seconds.

Cook the spinach: Add fresh spinach in batches, letting it wilt. Cook off excess moisture. If using frozen, squeeze it very dry and stir it in to warm through.

Build the filling: Remove the skillet from heat. Stir in cooked rice or quinoa, feta, chopped herbs, lemon zest, a squeeze of lemon juice, and tomato paste or crushed tomatoes if using. Season with salt, pepper, and red pepper flakes. Taste and adjust brightness with more lemon or salt as needed.

Optional binder: If you want a firmer filling, let the mixture cool slightly and stir in the beaten egg.

Stuff the peppers: Spoon the filling into the peppers, packing it gently to the top. Place peppers upright in the baking dish. Drizzle with a little olive oil. If you like a saucier bake, pour 1/2 cup broth or water into the bottom of the dish.

Bake: Cover loosely with foil and bake 25 minutes. Uncover and bake another 10–15 minutes, until the peppers are tender and lightly blistered and the filling is hot.

Finish and serve: Let rest 5–10 minutes. Add a final squeeze of lemon, more herbs, or a sprinkle of feta on top. Serve warm.