Peel and cut the sweet potatoes into large chunks and place in a large sauce pan. Cover them with water and bring the pot to a boil
Boil until the sweet potatoes are fork tender and easily pierced with a fork. They should fall apart easily
Drain the sweet potatoes and add them back to the sauce pan. Mash them with a potato masher. Add the rest of the sweet potato filling ingredients: butter, brown sugar, salt, cinnamon, vanilla extract, and nutmeg and mix until well combined
In a separate bowl, make the pecan topping. Stir together the melted butter, brown sugar, all purpose flour, salt, and pecan pieces until combined
Grease a 9 by 13 baking dish with cooking spray
Add the sweet potato filling and spread it out evenly
Spread the pecan topping evenly over the top of the sweet potatoes
Bake the sweet potato casserole at 350 degrees for 25-35 minutes, or until bubbly. Watch it closely towards the end to avoid burning