Bring the water to a boil in a large saucepan over high heat, then add 2 teaspoons salt.
Whisking constantly, add polenta in a slow, steady stream and return to a boil. Reduce heat to a very low simmer. Cover partially; cook, stirring occasionally, until mixture is creamy and starting to pull away from the sides of the pan, about 40 minutes, adding sage in last 5 minutes. If polenta is too thick to stir, add more water (up to 1/2 cup), a little at a time, and continue cooking, stirring occasionally.
Remove from heat. Stir in butter, and season with pepper and more salt, as desired.
