First, make a slit down one side of each onion without cutting them in half. Boil the onions in salted water until soft, then separate the outer layers for stuffing and reserve the centers for the sauce.
Sauté diced onion and garlic in olive oil until fragrant. Add the ground beef and cook until browned. Stir in rice, tomatoes, herbs, and feta, then adjust seasoning with salt and pepper.
Place a spoonful of filling on each onion layer and roll carefully. Nestle them seam-side down in a baking dish so they stay intact during baking.
Blend tomato paste with onion broth and centers until smooth. Pour the sauce over the stuffed onions, covering them mostly. Season with salt and pepper.
Cover with foil and bake at 375°F for 1 hour. Remove foil and bake another 30 minutes until onions are caramelized. Broil for a minute if you want extra browning.