Preheat oven to 350°F. Grease a 9- by 5-inch loaf and dust with flour, or use baking spray. Line the bottom and longer sides with parchment paper. (If you do not have a loaf pan, feel free to use muffin tins lined with cupcake liners and sprayed with baking spray.) Wash the sweet potatoes, pierce the skin with a small paring knife and wrap loosely in foil, leaving tops exposed. Place on the middle oven rack and bake until fork tender, about 1 hour. Let cool. Remove skins and purée flesh in a food processor or blender until smooth. Measure out ¾ cup sweet potato purée and set aside.

Combine flour, baking soda, salt and spices in a small bowl. Whisk together sugar, oil, eggs, water and ¾ cup sweet potato purée until smooth. Add the dry ingredients to the wet. Mix until just combined. Fold in walnuts and cranberries. Pour into the prepared pan.

For the oat topping, combine brown sugar, oats, flour and melted butter and toss together until combined and crumbly. Sprinkle generously on top of the prepared batter. Bake for 1 hour, or until a toothpick comes out clean. If the top becomes dark too quickly, you can cover it with foil halfway through baking if needed. (Muffin baking time will depend on the size of the muffins; mini muffins will bake for about 15–20 minutes, large muffins will bake for 20–30 minutes.) Remove from oven and cool on a baking rack before removing from pan. Slice and enjoy!