In a medium saucepan, combine the rinsed quinoa and vegetable broth. Bring to a boil, then reduce heat to low, cover, and simmer for 15 minutes or until quinoa is fluffy.
While the quinoa is cooking, prepare the bell peppers. Slice the tops off and remove the seeds and membranes. Place them upright in a baking dish.
In a large bowl, combine the cooked quinoa, cherry tomatoes, cucumber, red onion, olives, feta cheese, olive oil, lemon juice, oregano, salt, and pepper. Mix well until everything is evenly combined.
Spoon the quinoa mixture evenly into each prepared bell pepper, pressing down gently to pack the filling.
Cover the baking dish with aluminum foil and bake for 25 minutes.
Remove the foil and bake for an additional 10-15 minutes, or until the peppers are tender.
Remove from the oven and let cool for a few minutes. Garnish with fresh parsley before serving.