Bring 3 3/4 cups water, 1/4 cup heavy cream and 1 teaspoon kosher salt to a boil; whisk in 3/4 cup instant polenta. Reduce the heat to medium; cook, stirring occasionally, until creamy, 10 minutes. Stir in 1 cup grated fontina. Saute 12 ounces sliced mixed mushrooms and 2 teaspoons chopped rosemary in butter until tender, 6 minutes; stir in 1¿4 cup heavy cream and season with salt and pepper. Cook until thickened; toss with chopped parsley. Serve over the polenta.