Preheat your oven to 375°F (190°C). Begin by slicing the tops off the bell peppers and removing the seeds and membranes. Arrange the peppers in a baking dish.
Rinse the quinoa under cold water and drain. In a medium saucepan, bring the vegetable broth to a boil. Add the quinoa, reduce the heat to low, cover, and simmer for 15 minutes or until the liquid is absorbed. Remove from heat and let it sit, covered, for 5 minutes.
While the quinoa cooks, heat the olive oil in a skillet over medium heat. Sauté the red onion and garlic until the onion is translucent, about 3-4 minutes.
Fluff the cooked quinoa with a fork and transfer it to a large mixing bowl. Add the sautéed onion and garlic, Kalamata olives, cherry tomatoes, feta cheese, dried oregano, salt, pepper, and parsley. Stir until all ingredients are well combined.
Stuff the quinoa mixture into the hollowed-out bell peppers, pressing down gently to pack the mixture.
Place the stuffed peppers in the preheated oven and bake for 35 minutes, or until the peppers are tender and the filling is heated through.
Serve the stuffed peppers warm, topped with a generous dollop of tzatziki sauce.