Prep your sweet potatoes: peel (if desired), and roughly dice each sweet potato into 1-2 inch chunks. Place them in a large pot, and add just enough cold water to cover the sweet potatoes by an inch. Bring to a boil, and cook for 10-15 minutes until fork tender. Drain and place sweet potatoes back into the pot.

Add half of the butter (save the remaining 1/4 cup for the topping), and the egg to the sweet potatoes and mix well to combine. Mash with a potato masher or an immersion blender, until sweet potatoes are smooth and creamy. Add them to a 13 x 9 inch casserole dish.

Preheat oven to 350 degrees Fahrenheit. Crush pecans, and add them to a small mixing bowl with the brown sugar and flour. Cut the butter into small cubes or mash with a fork, and add to the bowl. Mix with a fork until well combined. Add the topping in an even layer on top of the sweet potatoes.

Bake casserole for 25-30 minutes, or until topping has browned and potatoes have warmed through.

Serve hot. Refrigerate any leftovers in an airtight container for up to 4 days.