Preheat oven to 350°F. Grease a 12 count muffin tin and set aside.: Before any mixing, ensure the oven is hot and stable at 350°F . You should feel a warm wave when you open the oven door and see the racks ready. Preheating guarantees the muffins begin rising immediately, creating a tender interior. A common mistake is putting batter into a cold oven, which produces dense muffins, so wait until it reaches temperature. Greasing the tin ensures easy release, and it should glisten slightly when you run your finger over it.
In a medium bowl, combine flours, salt, cinnamon and baking powder.: As you whisk the dry mix, inhale the warm scent of cinnamon spreading through the bowl. This step distributes the baking powder and salt evenly so every muffin rises and tastes balanced. A clumpy dry mix can lead to uneven pockets of leavening, so break up any lumps with a fork. Avoid overworking the mixture; a simple whisk is enough to blend the ingredients uniformly.
In a large bowl, whisk coconut oil, brown sugar, eggs, oatmeal and applesauce. Add the dry ingredients and stir until just combined. Stir in chopped apple.: The batter becomes glossy and slightly thick when the coconut oil and eggs emulsify with the sugars. You will hear a faint scraping sound as you fold, and the aroma will be sweet and inviting. Mixing the dry ingredients in last keeps the crumb tender, because overmixing activates gluten and makes muffins tough. When you fold in the apple , ensure the pieces are evenly distributed, and stop once there are no streaks of flour visible.
Divide the batter between 12 muffin cups.: Spoon or scoop the batter so each cup is evenly filled, usually about three quarters full for nice domes. The batter should feel slightly thick and moist, not runny, and the muffin cups will appear evenly distributed in color and texture. Uneven batter leads to varying bake times, so use a scoop for uniformity. If you overfill, the muffins may spill over and bake unevenly, so aim for consistency.
In a separate bowl, combine flour, oats, brown sugar, walnuts and cinnamon. Add the butter. Using your fingers, work the butter into the dry ingredients until it resembles wet sand and clumps together when you press it together. Sprinkle the streusel over the batter in the 12 muffin cups.: The streusel should feel cool and crumbly, and working cold butter into the dry mix creates little tender pockets that melt during baking. When you press a pinch and it clumps, it is ready. As the mixture bakes, it will crisp to a golden hue and offer a satisfying crunch against the soft muffin. A common error is warming the butter too much, which makes the mix greasy and prevents that desirable crumbly texture.
Bake for 13-15 minutes until a toothpick inserted into the center comes out clean.: As you bake, watch the tops color to a light golden brown and smell the kitchen fill with cinnamon and apple notes. The sound is subtle, but you may hear a quiet settling as steam releases. Use a toothpick in the center to check doneness and look for a clean pull, with maybe a few moist crumbs but no raw batter. Overbaking will dry them out, so remove the muffins when they spring back slightly to the touch and the centers test clean.