- To make the casserole : Place the sweet potatoes in a large pot and cover with water. Bring to a boil and cook for 8 to 10 minutes, until tender. Remove from the heat and drain. Set aside.
- Preheat the oven to 350°F. Lightly spray a 9 × 13-inch baking dish with cooking spray.
- Transfer the sweet potatoes to a large bowl. Add the milk, maple syrup, sugar, butter, lemon juice, nutritional yeast (if using), cinnamon, ginger, salt, and nutmeg. Use a masher to mash and combine the mixture until mostly smooth.
- Transfer to the prepared baking dish.
- To make the topping : Mix together the pecans, oats, cornflakes, oat flour, sugar, cinnamon, and salt. Pour the melted butter and maple syrup over the top and stir until combined. Spread the topping over the casserole.
- Bake for 20 minutes, or until the topping is crispy and the casserole is heated through. Serve immediately. Leftovers will keep in an airtight container in the fridge for 3 to 4 days.
- VARIATION For a richer flavor, instead of boiling the sweet potatoes, roast them whole for 1 hour at 400°F . Let them cool, then scoop the flesh from the skins.