In a large bowl, combine flour, 1/2 cup sugar, baking powder, baking soda, 1/2 tsp cinnamon, nutmeg, and salt. Set aside.
In a small bowl, stir yogurt until creamy, then add beaten egg. Add melted butter and grated lemon peel. Pour into the flour mixture and stir quickly until just mixed. Gently fold in 3 peeled, diced baking apples and spoon into a greased muffin tins about two-thirds full.
Topping: Combine 1/4 cup sugar with 2 tsp cinnamon and sprinkle over the unbaked muffins.
Bake in a 375° F oven for 20 to 25 minutes, or until golden brown. Cool on a wire rack.Servings: 1 dozen muffins