Preheat oven to 450 degrees F. Pierce the sweet potatoes two or three times with a fork. Arrange them on a baking sheet and bake for 45 minutes to 1 hour, until the flesh is tender. Slit the skin to let the steam escape.

Heat the milk, butter, and maple syrup in a large skillet or saute pan; do not boil. Scoop the sweet potato flesh into the same pan, then mash mixture together with a potato masher. Mash in the lemon juice and salt and pepper, to taste; the texture should be slightly lumpy but still mashed. Serve right from the pan or transfer to a serving dish. Dust the top with cinnamon or nutmeg, and serve at once.