Peel and cut potatoes into 1 1/2-inch-thick slices. Place in a medium saucepan. Cover with cold water; add 1 tablespoon salt. bring to a simmer. If using a potato ricer, fill another saucepan with water; place over low heat. Keep potatoes at a low simmer until a knife slips in and out easily. Drain potatoes in a colander. Place milk in a small saucepan over medium-high heat.
If using an electric mixer with paddle attachment, proceed to Step 4. If using a potato ricer, place a heat-proof bowl or top of a double boiler over a pan of simmering water. Press hot, drained potatoes through ricer into bowl.
Stir potatoes with a wooden spoon until smooth, about 1 minute. Using a whisk, incorporate butter. Drizzle in hot milk, whisking continuously. Add pepper, nutmeg, and salt to taste; whisk to combine. Serve immediately.
For the electric-mixer method, transfer hot, drained potatoes to bowl of an electric mixer fitted with the paddle attachment. Mix on medium-low speed, until most lumps have disappeared, about 1 minute. Add butter; mix until blended. On low speed, add hot milk in a slow stream, then add pepper, nutmeg, and salt to taste. Mix to combine.