Preheat oven to 350°F (180°C). Spray 9 jumbo muffin pan wells with baking spray with flour, or line 9 wells with jumbo paper liners.

In a medium bowl, whisk together flour, baking powder, ¾ teaspoon (2.25 grams) salt, and nutmeg.

In a large bowl, whisk together granulated sugar, ½ cup (110 grams) brown sugar, and eggs. Whisk in oil and milk until well combined. Add sweet potato, and whisk until well combined. Add flour mixture, and fold until combined. Let stand for 30 minutes.

In another medium bowl, stir together walnuts, cinnamon, remaining ½ cup (110 grams) brown sugar, and remaining ¼ teaspoon salt.

Spoon batter into prepared muffin cups, filling halfway full (about ¼ cup [70 grams] each). Make a well in centers, and spoon 1½ tablespoons (23 grams) walnut mixture into each well, leaving ⅛-inch border around edges. Divide remaining batter among wells. Sprinkle with Oat-Walnut Streusel.

Bake until a wooden pick inserted in center comes out clean, about 30 minutes. Let cool in pan for 10 minutes. Remove from pan, and let cool on a wire rack. Serve warm or at room temperature.