Scrub the russet potatoes well, and peel them if you like. Cut the potatoes into large chunks and place them in a very large pot. Cover the potatoes with water, adding a few extra inches of water on top. Generously salt the water, then bring to a boil over high heat. Adjust heat and vigorously simmer the potatoes until they are tender when pierced with a knife, about 20 minutes.

While the potatoes are cooking, warm 3 cups of lactose-free whole milk or half-and-half together with 1/2 cup unsalted butter until both are melted and hot, but not boiling. Butter the inside of your slow cooker to prevent sticking.

Once the potatoes are tender, drain them thoroughly in a colander. Return the potatoes to the pot over low heat and toss them gently to let excess moisture evaporate, which will yield fluffier mashed potatoes.

Depending on your preferred texture, either mash the potatoes in the pot with a hand masher or run them through a ricer or food mill directly into the slow cooker. Gradually mix in the warmed milk and melted butter from Step 2, adding more milk as needed to achieve a slightly looser texture—mashed potatoes will thicken a bit as they sit. Season generously with salt and black pepper to taste. I like to taste as I go for the perfect seasoning balance. If using a ricer or food mill, combine everything with a balloon whisk until smooth.

Transfer the mashed potatoes to the buttered slow cooker. Dot the top with the remaining 3 tablespoons of unsalted butter. Cover and set the slow cooker on warm; the mashed potatoes can be held for up to 4 hours. If you want to add an extra touch, I like to sprinkle a little extra black pepper on top before serving.