Combine all ingredients except the oats and brown sugar in a large bowl. Add oats and mix well.
Transfer mixture to a buttered baking dish. (You may use a 9-inch pie plate or an 9-inch square pan–or really any 2-quart baking dish–depending on how thick you would like your oatmeal. A 10-inch cast iron skillet works well, too.) Cover and refrigerate overnight or for at least 4 hours.
When ready to bake, let the baking dish sit on the counter while the oven pre-heats to 375 degrees. Uncover the oatmeal and bake for 27-30 minutes or until just firm in the center. Remove oatmeal from oven, set oven to broil, and sprinkle brown sugar over top of oatmeal. Broil for a minute, watching very closely, or until the topping is melted and lightly browned.
May serve with warmed milk, slivered almonds, chopped walnuts or pecans, and sliced bananas, if desired. My kids eat it plain, like a muffin; I load it up. Cover and store leftovers in the refrigerator. May reheat gently in the microwave.