Step 1: Preheat your oven to 400°F (200°C) and line a 12-cup muffin tin with paper liners or grease with non-stick spray.
Step 2: Prepare the streusel by mixing the walnuts, 1/4 cup flour, 1/4 cup brown sugar, and 1/2 teaspoon cinnamon in a small bowl. Cut in the 2 tablespoons of cold butter with a fork until crumbly. Set aside.
Step 3: In a large bowl, whisk together the 2 cups of flour, granulated sugar, 1/4 cup brown sugar, baking powder, 2 teaspoons cinnamon, nutmeg, and salt.
Step 4: In a separate medium bowl, beat the eggs lightly, then whisk in the melted butter, milk, and vanilla extract.
Step 5: Make a well in the center of the dry ingredients and pour in the wet ingredients. Fold gently with a spatula until just combined; do not overmix.
Step 6: Carefully fold in the diced pears until evenly distributed through the batter.
Step 7: Scoop the batter into the prepared muffin tin, filling each cup nearly to the top.
Step 8: Sprinkle the walnut streusel generously over each muffin, pressing down slightly so it sticks.
Step 9: Bake at 400°F for 5 minutes, then reduce the heat to 375°F (190°C) and bake for another 15-18 minutes, or until a toothpick comes out clean.
Step 10: Allow muffins to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.