Preheat oven to 375 degrees F. Line muffin pans with paper liners or coat with cooking spray. Set aside.: The kitchen fills with a faint warm scent as the oven approaches the right heat, and that steady background heat is crucial for an even rise. Preparing liners or greasing the pan prevents sticking, and setting everything aside keeps you from rushing once the batter is ready. A common slip here is turning the oven on too late, which can cause uneven baking, so give the oven time to reach temperature fully before inserting the tray.
In a large bowl, whisk together flour, sugars, baking powder, salt, cinnamon, and nutmeg. Set aside.: Dry mixing distributes leavening and spices uniformly, and you should see a homogenous, slightly speckled blend where cinnamon and nutmeg are dispersed. This step ensures each muffin gets the same lift and spice balance, which affects both flavor and crumb. Avoid overworking the dry mix into a compact mass, which can lead to uneven incorporation when you add liquids.
In a small bowl, whisk together buttermilk, butter, honey, egg, and vanilla. Pour the liquid mixture over the dry ingredients and stir until flour disappears. Gently fold in the peaches.: The wet mixture smells rich and slightly tangy from the buttermilk , and when you combine it with the dry ingredients you should stop mixing once the streaks of flour vanish to keep the muffins tender. Folding in the peaches gently preserves their shape and prevents color streaking through the batter. A typical mistake is overmixing, which makes the crumb tough, so fold with a light hand until the fruit is evenly distributed.
In a small bowl, combine oats, flour, brown sugar, cinnamon, and butter. Mix topping together with your fingers until you have a crumbly mixture. Stir in the honey and mix until combined.: As you rub the cold butter into the oats and dry ingredients, the mixture will transform into irregular crumbs, emitting a toasty, buttery aroma. The texture should be loosely clumped, not a uniform paste, so the bits crisp up in the oven. Stirring in the honey binds the crumb slightly and adds gloss, but be careful not to overwork the topping into a dense ball, which would prevent the desired crunchy finish.
Fill muffin cups 3/4 of the way full. Sprinkle each muffin with oat streusel topping. Bake for 18-20 minutes or until a toothpick stuck in the middle comes out clean. Let cool for 5 minutes. Serve and enjoy!: When filling the cups you can hear the soft plop of batter landing and see domes begin to form as they bake. The streusel should sit on top as loose crumbs that toast to a golden brown and smell nutty and sweet. Bake until a toothpick comes out clean to ensure the center is set, and resist the urge to overbake which dries the muffins. A frequent oversight is overcrowding the oven, which can change bake time and lead to uneven browning, so give the pans room to circulate heat.