In a large heavy saucepan combine the water and the broth and bring the mixture to a boil. Add the salt and polenta in a stream, whisking, and simmer the mixture, stirring, for 4 to 5 minutes or until it is thick. (The polenta will splatter.) Stir in 3/4 cup of the Parmesan. Working quickly, spread the polenta evenly in a lightly oiled shallow gratin or lasagna pan and let cool completely. Cover and chill the polenta for 30 minutes or until very firm and then invert onto a cutting board and cut into 8 squares. The polenta may be made up to this point a day ahead and kept covered and chilled. In a large skillet cook the onion and the garlic in 3 tablespoons of the oil over moderately low heat until softened. Add the exotic and white mushrooms and cook over moderate heat, stirring until almost all the liquid the mushrooms give off has evaporated. Add the Madeira and simmer the mixture until most of the Madeira is evaporated. Stir in the thyme and salt and pepper to taste and keep the mixture warm. In a large nonstick pan heat the remaining 2 tablespoons oil over moderately high heat. Add the polenta squares and saute them until they are heated through and nicely browned on both sides. Arrange two polenta squares decoratively on each of 4 plates and divide the mushroom mixture among the plates. Sprinkle each serving with some of the additional Parmesan and garnish each plate with a thyme sprig. Serves 4 Adapted from a Gourmet Magazine recipe. NOTES : From GMA Food Correspondent Sara Moulton Recipe by: Good Morning America