Preheat the oven to 375°F.
Cut the tops off the onions and carefully scoop out the insides, leaving about 1/2 inch of the onion shell. Chop the scooped-out onion and set aside.
In a skillet, heat the olive oil over medium heat. Add the chopped onion and garlic, and sauté for about 5 minutes until softened.
Stir in the cooked quinoa, diced tomatoes, feta cheese, parsley, olives, oregano, salt, and pepper. Mix well and remove from heat.
Stuff each onion shell with the quinoa mixture, pressing down gently to pack it in.
Place the stuffed onions in a baking dish and pour the vegetable broth around them. Cover the dish with aluminum foil.
Bake in the preheated oven for 30 minutes.
Remove the foil and bake for an additional 15 minutes, or until the onions are tender and slightly caramelized.
Let cool for a few minutes before serving.