First, make the Mashed Potato. We need thin mashed potato, so add more milk (about 2 cups) and less salt, because hotdogs are kind of salty.
Meanwhile bring a pot of water to a boil. Add the hot dogs. Reduce heat to medium-low and cook for 5-6 minutes. Remove hot dogs, drain and thinly slice (or grate) them.
Preheat the oven to 350 degrees F. In a large bowl, mix the mashed potato and sliced hot dogs. Grease an 8x11-inch baking dish with canola oil. Spoon the potato mixture into the baking dish and sprinkle with cheese.
Place on the middle rack of the oven and bake for 15 minutes. Then broil on low heat until golden brown, about 2-5 minutes (keep an eye on casserole while broiling). Remove the sausage & potato casserole from oven and serve immediately.