Grease a 12-inch cast iron skillet or 9×13 baking dish.
In a large mixing bowl, combine all ingredients except the oats, mascarpone, and brown sugar. Stir in the oats and combine well. Transfer the mixture to the prepared skillet and spread evenly around. Dot the mascarpone cheese evenly across the top, and then cover tightly with plastic wrap or foil and refrigerate overnight.
In the morning, preheat the oven to 375 degrees. Allow the oatmeal to sit on the counter while the oven is preheating. Uncover the oatmeal and bake for about 35 minutes or until just firm in the middle.
Remove from the oven, switch oven setting to broil, and sprinkle brown sugar over the top. Broil oatmeal for one minute, watching closely, or until the sugar is melted and top is golden.