Preheat the oven to 350°F. Grease an 8x8 inch baking dish with nonstick spray.
Place the sweet potatoes in a large pot, and cover with two inches of cold water. Add a big pinch of salt, and bring to a high simmer over high heat.
Reduce the heat to maintain the simmer, and cook the potatoes for 8 to 10 minutes, or until very tender. Drain the potatoes and return them to the pot.
Use a potato masher or immersion blender to mash the sweet potatoes until very smooth.
Add the butter, sugar, cinnamon, crushed red pepper (if using), salt, and pepper. Stir until well combined. Fold in half of the pecans.
Transfer the sweet potato mixture to the prepared baking dish, and smooth into a flat layer. Sprinkle the remaining pecans over the top, followed by the marshmallows.
Bake on the center rack of the oven for 10 to 15 minutes, or until the marshmallows are melted and golden-brown, and the potatoes are heated through. Rotate the pan after the first 5 minutes for even cooking.
Remove from the oven and let stand for 5 to 10 minutes before serving.