Light and tender, these apple muffins have a sweet streusel topping that makes them extra-special. Diced pears also work well, or try a combination of apples and pears.
Position a rack in the middle of the oven and preheat to 375 degrees F. Line 2 muffin pans with paper liners and set aside.
To make the streusel topping, combine the sugar, flour, and cinnamon in a small bowl and whisk to blend. Using 2 knives or your fingers, cut the butter into the flour mixture until it's in pieces the size of small peas. Stir in the nuts. Set aside.
Combine the flour, sugar, baking powder, salt, baking soda, and cinnamon in a large bowl and whisk to blend.
In another medium bowl, whisk the eggs, then add the yogurt and oil, and whisk to combine. Stir in the apples.
Add the yogurt-apple mixture to the dry flour mix, stirring just to combine. Overmixing will cause the muffins to be tough rather than light and tender. The batter will be thick.
Using an ice cream scoop or large spoon, divide the batter between the paper-lined muffin cups, filling each almost to the rim. Sprinkle some of the streusel topping on each muffin.
Bake until the muffins are golden brown and a toothpick inserted into the center comes out clean, about 25 minutes.
Allow the muffins to cool slightly before removing them from the pan. Once completely cool, wrap tightly and store at room temperature for up to 2 days.