Preheat oven to 180°C / 350°F. Line 10 muffin cups with muffin cases, or grease the tins.
Prepare the peaches. Slice each in half and twist to remove from the stone. Dice one peach into small (about 1cm) dice. The other, thinly slice. Set aside.
Sift oat flour, all purpose flour, whole wheat pastry flour, brown sugar, baking powder, baking soda, and salt together into a medium bowl.
In a small bowl combine melted butter, milk, sour cream, and the egg and whisk well to combine. Pour the wet ingredients into the dry, and mix until just barely combined.
Fold in the diced peach.
Spoon the batter evenly into 10 muffin cases, and top each with one or two thin slices of peach.
Bake for 25-30 minutes, rotating the pan once. The muffins should be slightly puffed and golden on top, and a toothpick inserted towards the center should come out clean.
Remove the muffins to a wire rack to cool. Serve warm or at room temperature.