Cut off the tops and bottoms of the onions and peel them. Make a slit down one side of each onion, being careful not to cut through the layers. Boil the onions in salted water for about 10-15 minutes until softened. Reserve 4 cups of the onion broth for the stuffing and sauce. Drain and let cool. Gently separate the layers, keeping them intact for stuffing.

In a skillet over medium heat, add the chopped onion and olive oil. Cook until softened, about 8 minutes. Add the grated garlic and warm through until fragrant. Stir in the ground beef, seasoning with salt and pepper. Cook until the beef is no longer pink. Add 1 cup of the tomato puree and cook until thickened. Then, add the rice and 1 and 1/2 cups of the reserved onion broth. Cook for 6-8 minutes or until the broth is absorbed. Be careful not to overcook the filling. Remove from heat and mix in the herbs. Set aside.

Take a layer of onion and place 1-2 spoonfuls of the filling in the center. Roll the onion around the filling to form a stuffed 'packet.' Repeat with the remaining layers and filling.

Place the stuffed onions seam-side down in a baking pan. Arrange them snugly in a single layer to prevent unrolling during cooking.

In a bowl, mix the sauce ingredients and pour the sauce over the stuffed onions. Cover the pan with foil or parchment paper.

Preheat the oven to 400°F (200°C). Bake for 45 minutes. Uncover the pan and bake for an additional 35-45 minutes until the onions are tender and caramelized. For a nice finish, set the oven to the Broil setting for the final 5 minutes of baking.

Let the stuffed onions cool slightly before serving. Garnish with extra parsley, if desired. Serve with feta cheese and yogurt on the side.