Preheat the oven to 375°F (190°C) and lightly grease a 12-cup muffin pan.
Mash 1 ripe banana in a large bowl. Add 1 cup Greek yogurt, ¼ cup honey, 1 large egg, and 1½ teaspoon vanilla extract, then whisk until smooth.
Stir in 1½ cups rolled oats, ½ teaspoon cinnamon, optional pinch of nutmeg, and 1 teaspoon baking powder until no dry oats remain.
Fold in 1 heaping cup diced peach (leave the skin on). Divide the batter evenly between the muffin cups, filling each almost to the top. Top with a peach slice and chopped pecans, gently pressing them into the batter.
Bake at 375°F (190°C) for 25 minutes, or until lightly golden. Cool in the pan for 30–40 minutes, then loosen with a small knife and lift out.
