Preheat your oven to 375°F (190°C). While it's heating, line a baking dish with parchment paper.
Heat olive oil in a large skillet over medium heat. Add diced onions and cook until translucent, about 5 minutes.
Add minced garlic and cook for another minute until fragrant.
Stir in the chopped spinach and cook until wilted, approximately 2-3 minutes.
Remove from heat and mix in the cooked quinoa or rice, drained tomatoes, crumbled feta, oregano, salt, and pepper.
Arrange the halved peppers in the prepared baking dish, cut side up.
Divide the filling mixture evenly among the pepper halves.
Cover the dish with foil and bake for 35-40 minutes.
Remove foil and bake for an additional 10 minutes until peppers are tender and the top is lightly golden.