Preheat your oven to 375°F and grease a 9x13-inch baking dish.
Wash and peel the sweet potatoes. Cut them into 1-inch cubes and place them in a large pot.
Fill the pot with enough water to cover the sweet potatoes and bring it to a boil. Cook for about 10-15 minutes or until the sweet potatoes are fork-tender.
Drain the sweet potatoes and transfer them to a large mixing bowl. Mash them with a potato masher or fork until smooth.
In the same bowl, add the pure maple syrup, unsweetened almond milk, vanilla extract, egg, cinnamon, nutmeg, and salt. Mix everything together until well combined.
Transfer the sweet potato mixture to the greased baking dish, spreading it evenly.
Bake in the preheated oven for 20-25 minutes, or until the edges are slightly golden.
Remove the casserole from the oven and sprinkle the mini marshmallows and pecans on top.
Return the casserole to the oven and broil for 1-2 minutes, or until the marshmallows are lightly browned and toasted. Keep a close eye to avoid burning them.
Remove from the oven, let it cool for a few minutes, and serve your sweet potato casserole. Enjoy!