Preheat the oven to 375 degrees. Lightly grease a 9x13-inch casserole dish or other 3-quart casserole dish and set aside.
Prick the sweet potatoes all over with a fork, then place them on a baking sheet lined with foil or a silpat mat. Bake until the potatoes are fork tender, about 1 hour or so, depending upon the size of your potatoes. Remove from the oven and let sit until cool enough to handle, about 5 minutes. Peel and discard the skins (they should come off easily with your fingers), break the potatoes in large chunks with a fork, then place the chunks into the bowl of a standing mixer fitted with the paddle attachment or a large mixing bowl.
While the potatoes bake, pour the almond milk in a small saucepan. Split the vanilla beans, scrape the seeds into the pan, then add the empty pods to the pan too. Bring to a very gentle simmer over medium heat for 5 minutes, stirring occasionally. Do not let the almond milk boil. Remove from heat and let the vanilla steep while the potatoes continue baking.
Remove the vanilla bean pods from the saucepan, then pour the almond milk and vanilla bean specs into the bowl with the sweet potato chunks. Add the melted coconut oil, cinnamon, salt, nutmeg, and white pepper.
Mash the potatoes together with the milk and spices (or beat gently with a mixer or your paddle attachment or run though a food mill) until fairly smooth with some texture remaining. Taste and add additional salt/pepper as desired. Transfer to the prepared baking dish. With the back of a rubber spatula, press and smooth into a single layer.
In a separate bowl, combine the topping ingredients—oats, pecans, almond meal, cinnamon, salt, coconut oil, and maple syrup—until moist and evenly mixed. Sprinkle over the sweet potatoes.
Bake until the topping is toasted and fragrant and casserole is warmed through, about 20 minutes. Serve warm.
*You can substitute 2 tablespoons pure vanilla extract for the vanilla beans, but the flavor will not be as rich and you lose the pretty specks. **To make your own almond meal, pulse whole, raw almonds in the food processor until they form fine crumbs. Do not over process, or you will have almond butter instead. Make it ahead: Casserole can be prepared up through Step 5 one day in advance. Store refrigerated with plastic wrap over the top of the dish. Let come to room temperature prior to baking.Pecan oat topping adapted from my Cherry Blackberry Crisp