Preheat the Oven: Preheat your oven to 375°F (190°C).
Boil Sweet Potatoes: In a large pot, bring salted water to a boil. Add your diced sweet potatoes and cook until they are fork-tender, about 15 minutes. Drain and let cool slightly.
Mash Sweet Potatoes: In a large bowl, combine the boiled sweet potatoes with melted butter, brown sugar, maple syrup, vanilla extract, cinnamon, nutmeg, and salt. Mash until smooth and creamy.
Spread in Baking Dish: Transfer the sweet potato mixture to a greased 9x13 inch baking dish, smoothing the top with a spatula.
Add Toppings: Evenly spread the cranberry sauce over the sweet potato mixture. Then, sprinkle mini marshmallows and chopped pecans on top.
Bake the Casserole: Place the casserole in the preheated oven and bake for 25-30 minutes, or until the marshmallows are golden brown and the edges are bubbling.
Garnish and Serve: Once out of the oven, let it sit for about 5 minutes. Garnish with fresh rosemary if desired, and serve warm as a delightful side dish at your Thanksgiving table.