Add about ¼ cup of the pasta water to the pan, at which point the pasta should be about ready. Lift it straight from the pot with a pasta spoon and toss into the pan with the mushrooms. Toss the spaghetti about to coat it in the woodsy sauce, letting it absorb most of the liquid to take it to the perfect al dente bite—about 2 – 3 minutes. What you’ll be left with is a scant, but flavorsome, starchy liquid. Shut off the heat and stir in the butter, letting it melt, creating a luxurious and velvety sauce.