- Place the sweet potatoes on a baking sheet and pop them into a preheated 400 degrees F oven for 60 to 70 minutes, or until they are tender. When the potatoes are cool enough to handle, scrape out the insides and use an electric mixer on high speed to beat the potatoes until they are mashed and smooth. Measure exactly 6 cups of the mashed sweet potatoes into a large bowl.
- Add the brown sugar, heavy cream, melted butter, cinnamon, and salt into the sweet potatoes and mix well with the beater until all the ingredients are incorporated. Pour this mixture into an 8 x 8-inch baking dish.
- Preheat oven to 350 degrees F.
- Make the oatmeal streusel by grinding the rolled oats to a coarse flour using a food processor. Make sure there are still visible bits of oats in there. A blender will also work.
- Combine the oat flour with the brown sugar, flour, and cinnamon in a small bowl. Cut the cold butter into the dry mixture using a pastry knife or a fork. You should have a crumbly mixture with pea-size bits. Sprinkle this oatmeal streusel over the sweet potato mixture and pop it into the oven for 50 to 60 minutes, or until the top begins to brown slightly.
- Remove the casserole from the oven and immediately spread the marshmallows over the top. Let this sit for about 10 minutes. The heat from the casserole will melt the marshmallows, and then it is ready to be served.
Makes 6 to 8 servings.