Preheat oven to 425 F. Lightly grease 15-1/2 x 10-1/2-inch baking sheet.
Cook rice according to directions.
In a nonstick skillet, heat oil over medium heat. Add onion and sauté until translucent, 8 to 10 minutes. Add ground beef, half of the mint, cinnamon, and season with salt and pepper to taste. Cook until ground meat is cooked through and browned, 6 to 8 minutes, breaking up the meat into small bits with cooking spoon. Remove from heat, and stir in remaining mint. Check and adjust seasoning.
Cut each tomato in half, horizontally. Using a scoop or spoon, scoop out tomato pulp and seeds from each half. Reserve 1 cup of chopped tomato pulp in a bowl. Place tomatoes, cut side up, on baking sheet.
In a large bowl, combine cooked rice, meat mixture, reserved tomato pulp, and feta cheese. Stir to blend. Fill each tomato half with 1/2 cup of the rice and meat mixture. Sprinkle panko across the tops of the filled tomatoes. Bake in preheated oven 20 to 25 minutes or until panko is toasted and filling is heated trough. If desired, garnish with additional chopped mint. Makes six servings.