Preheat oven to 180°C. Spray a 12-cup standard muffin tin with cooking spray; set aside.
Cut 3 apple quarters into small dice; cut remaining apple quarter into 12 thin slices for garnish.
Whisk together sugar, flour, baking soda, salt, and cinnamon in a medium bowl. Add diced apple and walnuts, if using; toss to coat.
Whisk together eggs, buttermilk, and butter in a small bowl. Gently fold buttermilk mixture into flour mixture until just combined; do not over mix.
Divide batter among muffin cups, filling each about three-quarters full. Top each with an apple slice. Bake until muffins are brown around edges and spring back when touched, 16 to 18 minutes.
Let muffins cool slightly, about 5 minutes, before turning out of tin onto a wire rack. Serve warm or at room temperature.
For more wonderful recipes from Ally, visit her blog Simply Delicious