Slice the tops off the tomatoes and gently scoop out the insides using a spoon. Leave a little flesh attached to form a shell. Reserve the insides for later use.
In a medium saucepan, combine the rinsed quinoa and vegetable broth. Bring to a boil over high heat, then reduce to a simmer, cover, and cook for about 15 minutes or until the quinoa is fluffy and the broth is absorbed.
While the quinoa is cooking, heat olive oil in a skillet over medium heat. Add the chopped red onion and bell pepper, and sauté for about 5 minutes until softened.
In a mixing bowl, combine the cooked quinoa, sautéed vegetables, diced cucumber, chopped olives, crumbled feta, parsley, lemon juice, salt, and pepper. Stir well to combine.
Spoon the quinoa mixture into the hollowed-out tomatoes, packing it down lightly.
Place the stuffed tomatoes in a baking dish and drizzle with a little olive oil.
Bake in the preheated oven for 20-25 minutes until the tomatoes are tender but still hold their shape.
Remove from the oven and let cool for a few minutes before serving.
Notes
Feel free to customize the vegetables based on your preference.