Preheat your oven to 425°F and line a 12-cup muffin tin with paper liners. While the oven heats, gather all your ingredients and ensure the egg, butter, and sour cream are at room temperature—this helps them emulsify properly with the sourdough discard for a tender crumb. I find that room-temperature ingredients mix more smoothly and create a better texture in the finished muffins.

In a large bowl, whisk together the flour, sugar, baking powder, baking soda, salt, and ground nutmeg. Whisking instead of stirring ensures the leavening agents are evenly distributed throughout the flour, which promotes uniform rise and a consistent crumb structure. Set this bowl aside.

In a separate bowl, whisk together the melted and cooled butter, sourdough discard, sour cream, room-temperature egg, vanilla extract, and lemon zest until fully combined. The sourdough adds a subtle tang that makes these muffins special. While you're finishing the wet mixture, gently toss the blueberries in a spoonful of flour to coat them lightly—this prevents them from sinking to the bottom during baking.

Pour the wet ingredient mixture from Step 3 into the dry ingredients from Step 2. Using a spatula or wooden spoon, fold everything together gently just until no streaks of flour remain—do not overmix, as this activates gluten and creates tough, dense muffins. Fold in the flour-coated blueberries as your final action, being careful not to crush them.

Divide the batter evenly among the lined muffin cups, filling each about three-quarters full. Using an ice cream scoop or two spoons makes this much faster and more consistent. Sprinkle a small pinch of turbinado sugar over the top of each muffin—the coarser crystals create a crunchy, attractive topping that won't dissolve during baking.

Place the muffin tin in the preheated 425°F oven and bake for 8 minutes to give the muffins a strong initial rise. After 8 minutes, reduce the oven temperature to 350°F and bake for an additional 12 minutes until a toothpick inserted into the center of a muffin comes out clean or with just a few moist crumbs. The high-heat start sets the structure quickly, while the lower temperature finish allows the centers to bake through without overbrowning the tops.

Remove the muffin tin from the oven and let the muffins rest in the tin for 10 minutes—this allows them to set slightly without being fragile. Transfer the muffins to a wire cooling rack to cool completely. Serve warm or at room temperature; they'll stay tender and fresh for up to 2 days when stored in an airtight container.