Peel potatoes, rinse, and cut into 2-inch chunks. Combine potatoes and about 2 quarts water in a 5- to 6-quart pan. Cover and bring to a boil over high heat; reduce heat. Simmer until potatoes mash very easily, 20 to 25 minutes. Drain potatoes and return to pan.
Add cheese and milk to potatoes. Mash with a potato masher or a mixer until smooth. Season to taste with salt and pepper. Let cool to room temperature, about 40 minutes.
Put 1/2 the mashed potatoes in each of 2 foil pans (8 to 9 inch square). Seal pans with foil, and freeze (see Cold Facts below).
Thaw 1 pan (see notes). Bake, covered, in a 375° oven until potatoes are steaming, about 40 minutes (about 30 minutes in a convection oven).
Cold Facts. Packaging: freezer bags and foil pans. Squeeze out all air and seal bag. For better protection of frozen foods during storage, seal each bag inside a second one. Lay bags flat in freezer until solid so they will stack. Seal foil pans with a double layer of foil to protect foods for storage. Label each dish with permanent marker. Make a master list of the dishes. As you remove each from the freezer, cross it off the list. For best quality, freeze meat mixtures no longer than six weeks, others three months.
Note: Nutritional analysis is per serving.