Prep the peppers: Heat your oven to 375°F (190°C). Slice the tops off the peppers and remove seeds and membranes.
Lightly brush the outsides with olive oil and place them upright in a baking dish.
Soften the peppers (optional): If you like softer peppers, pre-bake them for 10 minutes while you make the filling. This step is optional, but it shortens the final bake time and gives a tender bite.
Sauté aromatics: Warm 2 tablespoons olive oil in a large skillet over medium heat. Add onion with a pinch of salt and cook 4–5 minutes until translucent.
Stir in garlic and cook 30 seconds until fragrant.
Cook the spinach: Add the chopped spinach in batches, stirring until wilted. If using frozen spinach, add it now and heat through. Season with a little salt and pepper.
Build the filling: Turn off the heat.
Stir in the cooked rice (or quinoa/couscous), feta, Greek yogurt, lemon juice, lemon zest, oregano, and red pepper flakes. Add parsley or dill. Taste and adjust with more salt, pepper, or lemon as needed.
The filling should be creamy and well-seasoned.
Stuff the peppers: Spoon the filling into each pepper, packing it gently to the top. Sprinkle with Parmesan if using.
Add moisture to the pan: Pour 1/4 cup water or broth into the bottom of the baking dish. This helps steam the peppers and keeps them tender.
Bake: Cover loosely with foil and bake 20 minutes.
Remove foil and bake another 10–15 minutes, until the peppers are tender and the tops are lightly golden.
Finish and serve: Let rest for 5 minutes. Garnish with extra herbs and a squeeze of lemon. Serve warm.