Preheat oven to 375 degrees F

Peel potatoes, then uniformly slice using a mandoline or knife. Place in a large bowl.

In a small saucepan, over low/medium heat, add the milk or cream, brown sugar, 1 tsp cinnamon, and 1/2 tsp nutmeg. Stir until sugar has dissolved, about 3-5 minutes. Remove from heat and pour over the sweet potatoes in large bowl. Mix to coat all the sweet potatoes and allow to sit for 20 minutes to absorb.

In a casserole dish, spray non-stick cooking spray and arrange the sweet potatoes so they are fanned out almost vertically, throughout the dish. Discard the dairy mixture. Sprinkle the 1/2 tsp of cinnamon over the slices, scatter slices of butter on top, cover with foil and bake for 1 hour. 

While the sweet potatoes bake, in a large bowl combine all the topping ingredients until it has a crumbly texture (I typically use my hands to mix it and achieve the crumbly texture). Once the sweet potatoes are done cooking, remove from the oven, uncover, and sprinkle the brown sugar pecan mixture on top. Put back in the oven for 25 more minutes or until the topping begins to brown. Remove from oven and serve.