Cut the top off of the tomatoes and clean out all of the pulp leaving about a 1/2-inch thick skin. Reserve pulp.
Preheat oven to 375 degrees F.
In a saute pan, heat the butter and add the chopped onion to the pan. Cook until the onions are golden brown, and add in the garlic. Add in the pulp from the tomatoes and stir in the rice. Add in 1/4 cup olive oil and a little water and simmer for about 5 minutes. Add more water if necessary. Season with salt and pepper. Stir in the parsley, pine nuts, and mint.
Fill the tomatoes and sprinkle the top with the grated Kasseri and a drizzle of the remaining olive oil. Place in a baking dish with a 1/2-inch of water in the bottom and bake for 1 hour.