Prepare the Tomatoes: Cut the tops off the tomatoes and scoop out the insides with a spoon, setting the pulp aside in a bowl. Be careful not to break the tomato shells.
Make the Filling: In a pan, heat olive oil over medium heat. Sauté the chopped onion and garlic until translucent. Add the rice and cook for 2-3 minutes, stirring frequently. Incorporate the reserved tomato pulp, parsley, mint, salt, pepper, and cinnamon, mixing well.
Stuff the Tomatoes: Carefully spoon the rice mixture into each tomato shell, packing it lightly. Leave a little space at the top as the rice will expand while cooking.
Bake: Place the stuffed tomatoes in a baking dish, adding a little water or tomato juice to the bottom of the dish to prevent sticking. Drizzle with more olive oil and cover with the tops. Bake in a preheated oven at 375°F (190°C) for about 40-45 minutes, or until the tomatoes are tender.
Serve: Allow the Gemista to cool slightly before serving. Optionally, top with crumbled feta cheese for an extra layer of flavor.