Peel and core the pears, then either finely dice them or grate on the large holes of a box grater.

Preheat the oven to 425 F. Grease a 12-cup muffin tin or line with paper liners.

Combine the flour, baking powder, baking soda, cinnamon, salt, and nutmeg in a large bowl; whisk to blend and set aside.

In a mixing bowl with an electric mixer, beat the butter and brown sugar on medium speed until light and fluffy, about 3 to 4 minutes. Scrape down the sides of the bowl from time to time.

Add the eggs, one at a time, beating well after each addition, then add the sour cream and vanilla. Mix until well blended.

With the mixer on low speed, gradually add the dry ingredients and milk until just blended. Avoid overmixing.

Fold the diced pears and walnuts, if using, into the batter.

Spoon the batter into the prepared muffin tin, filling each well about 2/3 to 3/4 full. If desired, sprinkle with turbinado sugar.

Place the muffins in the oven and bake for 5 minutes at 425 F.

Reduce the temperature to 350 F and continue baking for 18 to 24 minutes, or until the tops are golden brown and a toothpick inserted into the center of a muffin comes out clean.

Let the muffins cool for 5 minutes in the muffin tin, then remove them to a wire rack to cool.