Preheat oven to 350 degrees.
Spray a 9x13 inch baking casserole dish.
In a large bowl, mix sweet potatoes, butter, milk, sugar, vanilla, coconut and lemon extracts and eggs.
Spread mixture into the casserole dish.
Make the Streusel - In a small bowl, mix together brown sugar, graham cracker crumbs, and oatmeal.
Cut in cold butter until mixture is crumbly, then stir in nuts.
Sprinkle streusel over the sweet potatoes and top with marshmallows.
Bake for 25-30 minutes or until marshmallows are nicely browned.