While the polenta continues to cook, heat the pan of reserved oil on medium-high until hot. Add the mushrooms and cook, without stirring, 2 to 3 minutes, or until browned. (If the pan seems dry, add 2 teaspoons of olive oil.) Season with salt and pepper. Continue to cook, stirring frequently, 1 to 2 minutes, or until softened. Add the garlic and spinach; season with salt and pepper. Cook, stirring frequently, 1 to 2 minutes, or until the spinach has wilted. Turn off the heat. Add the vinegar (be careful, as the vinegar may splatter) and half the butter; stir to thoroughly combine. Season with salt and pepper to taste.