Preheat your oven to 375°F and grease a standard 12-cup muffin tin with butter or cooking spray. While the oven heats, measure out all your dry ingredients (oats, flour, baking powder, baking soda, cinnamon, nutmeg, and salt) into a medium bowl and whisk them together to distribute the leavening agents and spices evenly. Set the blueberries aside separately—if using frozen, do not thaw them as they'll hold their shape better in the batter.

In a separate bowl, melt the butter gently (about 30 seconds in the microwave or over low heat on the stovetop), then let it cool for just a minute. Add the room-temperature egg, yogurt, milk, honey, and vanilla to the melted butter and whisk together until smooth and well combined. The room-temperature egg is key here—it won't seize the butter the way a cold egg would, giving you a silky wet mixture.

Pour the wet ingredient mixture from Step 2 into the bowl of dry ingredients from Step 1 and gently fold together with a spatula or wooden spoon until just combined—there should be a few small lumps remaining. Overmixing develops gluten and creates tough, dense muffins, so stop as soon as you don't see streaks of flour. Fold in the blueberries last, being gentle to keep them intact.

Divide the batter evenly among the prepared muffin cups, filling each about two-thirds to three-quarters full. Bake for 14-24 minutes (start checking at 14 minutes, as baking times vary by oven) until a toothpick inserted into the center of a muffin comes out clean with just a few moist crumbs. The tops should be golden brown and slightly domed.

Remove the muffins from the oven and let them rest in the tin for about 5 minutes—this allows them to set slightly so they won't fall apart when unmolded. Run a thin knife around the edges of each muffin to loosen them, then gently transfer to a wire cooling rack. Serve warm or at room temperature.