Preheat your oven to 375°F (190°C). Slice the tops off the tomatoes and set them aside. Using a small spoon, carefully scoop out the pulp and seeds from the inside of the tomatoes, leaving about a 1/2-inch thick wall. Place the hollowed-out tomatoes in a lightly greased baking dish.
In a large mixing bowl, combine the cooked quinoa, crumbled feta cheese, olives, pine nuts, parsley, mint, and minced garlic. Drizzle with olive oil and lemon juice. Add the dried oregano and season with salt and pepper to taste. Stir everything together until the ingredients are well distributed.
Spoon the quinoa mixture generously into each hollowed tomato shell. Press down lightly with the back of the spoon to ensure they are packed full. You can pile the filling slightly above the rim of the tomato for a beautiful presentation.
Place the tomato tops back on if desired, or leave them off for a toasted top. Bake in the preheated oven for 20 to 25 minutes, or until the tomatoes are tender and the filling is heated through. Remove from the oven and let them rest for 5 minutes before serving.