Add pecans to a food processor and pulse into small crumbs.
Now add melted butter, brown sugar, cinnamon, nutmeg, and salt. Pulse 2-3 times to incorporate (don't over pulse or it will clump) Set aside.
Sweet Potatoes
Preheat your oven to 375 °F (190 °C). Lightly grease a 2-quart baking dish.
Place sweet potatoes in a large pot and cover with water. Bring to a rapid boil over high heat, then reduce heat to maintain a medium boil. Cook for 15-20 minutes until fork tender. Drain well, then place them back in the pot and mash slightly to release some steam.
Now add the butter, milk, maple syrup, vanilla, cinnamon, nutmeg, and salt. Continue to mash until well combined and the potatoes are fully broken down. Taste for seasoning and add more if needed.
Now place the sweet potato mash into your casserole dish in an even layer. Top with pecan crumble and bake for 20-30 minutes until heated throughout and topping is golden brown. (*optional: put 4-5 pieces of whole pecans on top of crumble). Enjoy!
Notes
I feel this casserole has the perfect amount of sweetness, but feel free to add more or less to suit your taste.