Preheat the grill to 400 degrees. In a mixing bowl, toss the tomatoes with the olive oil, salt and pepper. Place on the grill and cook for 2 to 3 minutes on each side. Remove from the grill and set aside. Preheat the oven to 400 degrees. Place the portobello mushroom on a parchment - lined baking sheet, cavity up. Drizzle both sides of the mushrooms with the olive oil. Season both sides with salt and pepper. Fan a quarter of the cheese over each cavity of the mushroom. Place in the oven and cook until the mushrooms are tender and the cheese is bubbly, about 10 minutes. Heat the olive oil in a large saute pan over medium heat. Add the onions. Season with salt and pepper. Saute until the onions are slightly soft, about 3 minutes. Add the water and garlic. Bring the mixture to a boil, reduce the heat to medium, and simmer for about 6 minutes. Add the rice and simmer, stirring constantly until the mixture is creamy and bubbly, about 18 minutes. Stir in the butter, cream, cheese, and green onions. Simmer for about 2 minutes, stirring constantly. Remove from the heat and stir in the tomatoes. To serve, slice each portobello into quarters. Spoon the risotto into each serving dish. Lay 2 slices of the portobello on top of the risotto. Garnish with parsley.