Transfer the peeled and cut sweet potatoes to a large pot and submerge them in cold water. Bring the pot to a boil and boil the sweet potatoes until they are fork tender for about 15 minutes or so. Drain them and dump them into a large mixing bowl. Add the butter and lightly mash them together with a fork. Then add the brown sugar, cinnamon, nutmeg, salt, milk and eggs. Use a hand mixer to whip them together thoroughly and make the mixture fluffy. Pre-heat the oven to 350 and get out a 9 x 13 baking dish. Spray it with cooking oil.

Turn out the sweet potato mixture into the pan and spread it out evenly, smoothing out the top. Then make the easy topping. Stir the walnuts, brown sugar, cinnamon, salt, melted butter and vanilla together thoroughly in a bowl until it is a sticky glaze. Evenly pour it over the sweet potato mixture, using a spoon to spread it out thinly. Bake the casserole for 45 minutes, until completely set. Let it cool for a couple of minutes, then serve immediately! It can also be assembled ahead the day before, covered and then baked the day of.