Whisk together sugar, flour, baking soda, salt, cinnamon, and nutmeg in a medium bowl. Add diced apples and walnuts, tossing to coat.
2
Whisk together eggs, yogurt, butter in a small bowl. Gently fold the liquid mixture into the flour mixture until just combined. Don''t over-mix or muffins will be tough. Divide batter among muffin cups.
3
In a small bowl, mix brown sugar with cinnamon. Sprinkle a little of the mixture over the top of each muffin.
4
Bake until muffins are brown around edges and spring back when touched, 16 to 18 minutes. Cool on rack for 5 minutes.
