Prepare sweet potatoes: Wash, peel, and cut sweet potatoes into chunks. Boil or steam until fork-tender (15-25 minutes), then drain and mash until smooth.
Make casserole filling: Add brown sugar, melted butter, eggs, vanilla extract, cinnamon, nutmeg, and cream to mashed sweet potatoes. Mix until smooth.
Prepare pecan streusel topping: In a bowl, combine brown sugar, flour, and salt. Mix in melted butter until crumbly, then stir in chopped pecans.
Assemble and bake: Preheat oven to 350°F (175°C). Grease a baking dish, spread filling, and top with pecan streusel. Bake for 30-35 minutes until bubbly and golden.
Cool and serve: Let the casserole rest for 10-15 minutes before serving.
Notes
Add mini marshmallows as a topping for an extra sweet twist or use coconut sugar for a caramel flavor.