POLENTA: In a large pot, over medium-high heat, bring water, 2 cups milk, and salt to a boil. Gradually whisk in polenta; reduce the heat to low and continue to cook, stirring frequently with a long wood spoon, until very thick and creamy, about 30 minutes. Remove from heat and stir in 3 tablespoons butter.
Line a 9 x 13-inch pan with plastic wrap. Evenly pour cooked polenta into the prepared pan. Cover with plastic wrap, laying it right on top of the polenta. Refrigerate for several hours until set or overnight.
The next day, cut the polenta into squares.
In a large nonstick skillet, over medium heat, add 2 tablespoons olive oil. Working in batches, add polenta squares, frying until golden brown on each side about 3-5 minutes. Keep warm.
MUSHROOMS: In a separate large skillet, over medium heat, add 1 tablespoon butter and 1 tablespoon olive oil. Add the onions and garlic; cook until softened about 3-5 minutes. Add mushrooms; cook until golden brown about 3-5 minutes.
GORGONZOLA SAUCE: Meanwhile, pour 2/3 cup milk into a saucepan and bring to a boil. Add Gorgonzola cheese, whisk until melted and smooth; whisk in sour cream and season with salt and pepper
Pour Gorgonzola sauce over mushrooms, stir to combine; spoon sauce over the fried polenta cakes. Sprinkle with chopped walnuts and serve.
Cook's Note: The Mushroom Gorgonzola Sauce would be awesome over a grilled steak!