Preheat the oven to 400 degrees F. Line a rimmed baking sheet with a silicone mat or parchment paper. (This will help with cleanup.) Spray a 2-quart shallow baking dish with nonstick cooking spray. Set aside.
Using a fork, pierce the sweet potatoes in several places and place on the prepared baking sheet. Bake until fork-tender, about 50 minutes. Remove to a rack to cool.
When the potatoes are almost tender, prepare the topping: In a small bowl, combine the chopped pecans, flour, Parmesan, baking powder, salt, and pepper. Stir to combine. Add the milk, oil, and sage. Stir until well combined. Set aside.
When the sweet potatoes are cool enough to handle, peel the potatoes, discarding the skin. Place the pulp in large bowl. Add the brown sugar and nutmeg. Season with salt and pepper. Smash the potatoes with a potato masher until chunky.
Transfer the sweet potatoes to the prepared baking dish. Lightly flour your hands and crumble the topping in small, cherry size pieces on top of the sweet potatoes. Transfer to the oven and bake until golden brown, about 30 minutes. Serve immediately.