Bring the water and salt to a boil in a medium saucepan. Whisk in the polenta, add the mushrooms and parsley. Stir constantly until it begins to thicken.
Reduce the flame and simmer for 10 minutes.
Pour the polenta into a shallow glass baking dish and smooth out to about 1/2 inch thick. Refrigerate 1 hour or until the polenta is firm. Slice the polenta into squares or rectangles.
To prepare the sauce, heat the oil in a skillet and sauté the garlic and onions for 2 to 3 minutes. Add the spaghetti sauce and basil. Cover, reduce flame and simmer for 5 to 7 minutes. While the sauce is cooking, pan fry the polenta.
Oil a skillet or griddle with oil and pan fry the slices on each side until golden and crisp. Place on a serving platter.
Dish out the tomato sauce over the polenta when serving.