In a 1-quart saucepan set over high heat, add the water and 1 tablespoon of oil and bring to a boil. While whisking, slowly add the polenta to the boiling water. Season the polenta with 2 teaspoons of the salt and lower the heat to a simmer. Continue to cook the polenta, partially covered and stirring often, until creamy and tender, about 20 to 25 minutes. Add 2 tablespoons of the butter to the polenta, stir to combine, and cover. Keep warm over very low heat while you prepare the mushrooms.

Set a large, 12-inch saute pan over medium-high heat. Add the remaining 6 tablespoons of butter and the remaining 2 tablespoons olive oil to the pan. Once the butter is melted, add the sage and mushrooms to the pan and cook until the mushrooms are wilted and most of the moisture has evaporated, about 10 minutes. Season with the remaining 1 teaspoon of salt and the pepper and garnish with the parsley. Serve over the polenta in warmed bowls.