Peel and halve 3 medium russet potatoes. Put in a microwave-safe bowl, cover with plastic wrap and make a small hole in the plastic with a knife. Microwave until tender, 12 to 15 minutes; let sit, covered, 2 minutes. Mash the potatoes with 1 to 1 1/4 cups hot milk, and salt to taste. Melt 3 tablespoons butter in a small skillet over medium heat and cook until golden brown; add 8 to 10 sage leaves and cook 30 more seconds. Pour over the potatoes.