Cut 1 inch from each end of onions. Stand onions on a flat side and make a cut straight down from top to bottom.

Bring a saucepan with 6 to 8 cups of salted water to a simmer. Add onions and simmer for 20 to 30 minutes until layers can be separated.

Remove onions and let cool, saving cooking water.

Heat olive oil in a skillet, cook half the beef until browned. Add diced onions, black pepper, salt, and cayenne, cooking until onions are translucent.

Stir in garlic, oregano, and tomato paste, cooking for a few minutes. Add rice and stir, then add vinegar and broth, cooking until rice absorbs liquid.

Remove from heat and let cool. Combine remaining beef, feta, parsley, mint, and oregano with cooled rice mixture. Refrigerate.

For tomato broth, combine onion cooking water, tomato sauce, and bouillon paste in a saucepan, simmering until needed.

Preheat oven to 375°F (190°C). Prepare onion sections for stuffing, adding filling to each and placing in a baking dish.

Cover with tomato broth, cover with foil, and bake for 1 hour. Remove foil and continue baking for another hour or until tender.

Let rest for 10 minutes before serving.