Lightly grease the inside of a 4- to 6-quart slow cooker with oil or butter.

Peel and slice the russet potatoes into thin rounds, about ⅛- to ¼-inch thick.

In a medium bowl or large measuring cup, whisk together the sweetened condensed milk, water, and kosher salt until smooth.

Layer the sliced potatoes evenly in the slow cooker crock, separating any pieces stuck together.

Pour the condensed milk mixture evenly over the potatoes, pressing down lightly so they are mostly submerged.

Cover and cook on LOW for 6 to 7 hours or on HIGH for 3 to 4 hours until the potatoes are fork-tender and the sauce has thickened.

Gently stir from the edges toward the center, adjust salt to taste, and let rest covered on WARM for 10 to 15 minutes before serving hot.