Preheat your oven to 375°F (190°C) and lightly grease a 9x13-inch baking dish with butter or non-stick cooking spray.
Place the cubed sweet potatoes in a large pot, cover with cold water, and bring to a boil. Cook for 15-20 minutes, or until the potatoes are very tender when pierced with a fork. Drain them thoroughly in a colander.
Transfer the drained sweet potatoes to a large mixing bowl. Use a potato masher or electric hand mixer to mash them until smooth with no lumps. Add the 1/4 cup of melted butter, maple syrup, beaten eggs, vanilla extract, cinnamon, nutmeg, milk, and salt. Mix with a spoon or spatula until all ingredients are fully combined and the mixture is smooth.
In a separate medium bowl, prepare the crumble topping by combining the chopped pecans, rolled oats, flour, and brown sugar. Add the 1/4 cup of cold, cubed butter and use your fingers or a pastry cutter to work it into the dry ingredients until the mixture resembles coarse, wet sand with some small pebbles of butter remaining.
Spread the sweet potato mixture evenly into the prepared baking dish. Sprinkle the pecan crumble topping evenly over the entire surface.
Bake for 25-30 minutes, or until the topping is golden brown and the sweet potato filling is bubbling around the edges. Let the bake rest for 10 minutes before serving to allow it to set.