Preheat the oven to 375°F with a rack in the middle position.

Bring a large pot of salted water to a boil. Trim the tops and bottoms off the onions, peel them, and cut a vertical slit down one side of each onion. Boil for 10 to 15 minutes, until the layers are pliable and tender. Save the onion broth. When the onions are cool enough to handle, gently separate the 3 or 4 largest outer layers from each onion and set them aside. Reserve the onion centers for the sauce.

Heat the olive oil in a large skillet over medium heat. Add the chopped onion and cook until soft, then stir in the garlic and cook for about 2 minutes, until fragrant.

Add the ground chuck and turn the heat to medium-high. Cook, breaking up the meat, until it begins to brown. Season with salt and pepper, stir in the crushed tomatoes, and let the mixture simmer until it starts to thicken. Add the arborio rice and mix well.

Remove the skillet from the heat and fold in the oregano, parsley, mint, and feta. Taste and add more salt and pepper if needed.

Scoop 1 to 2 tablespoons of filling onto the center of each onion layer and roll it up like a little package. Place seam side down in a 9 by 13 inch baking dish, packing the rolls very close together so they don't open. You may need two baking dishes to fit them all.

In a large measuring cup or bowl, combine the tomato paste, reserved onion broth, and chopped onion centers. Puree with an immersion blender until smooth. Taste and season with salt and pepper. Pour the sauce over the stuffed onions, adding more hot broth if necessary to mostly cover them. Cover the dish tightly with foil.

Bake covered for 1 hour. Remove the foil and bake for another 30 minutes, until the sauce has thickened and the onions have caramelized. If desired, broil for a couple of minutes, watching carefully.

Serve warm with pita and extra feta on the side.