Preheat your oven to 375°F (190°C). Slice the tops off the peppers and remove the seeds and membranes. Place the peppers in a baking dish.
Cook the rice in boiling salted water until just tender, about 10-12 minutes. Drain and set aside.
Heat olive oil in a skillet over medium heat. Add the sliced onions and cook, stirring occasionally, until they are golden brown and caramelized, about 10 minutes. This brings out their sweetness and depth of flavor.
Add the ground beef to the skillet with the onions, breaking it apart with a spatula. Cook until browned all over, about 8 minutes. Season with smoked paprika, salt, and pepper, then stir to combine. Remove from heat.
In a large mixing bowl, combine the cooked rice, seasoned ground meat mixture, crumbled feta cheese, and half of the cooked onions. Mix well to ensure everything is evenly distributed.
Spoon the filling into each hollowed-out pepper, pressing gently to pack the mixture. Place the filled peppers upright in the baking dish.
Sprinkle the remaining caramelized onions over the tops of the stuffed peppers for added flavor and presentation.
Cover the dish with foil and bake in the preheated oven for 30 minutes. Then, remove the foil and bake for an additional 10 minutes, until the peppers are tender and slightly charred around the edges.
Remove from the oven and let rest for a few minutes. Serve warm, garnished with extra feta or herbs if desired.