PREP: Preheat oven to 350°F (175°C). Generously coat a Muffin Tin with cooking spray. Measure and set out your egg and yogurt to reach room temperature. Melt coconut oil, measure and set aside to get to room temperature.
FLOUR AND DRY INGREDIENTS: In a large bowl, add the flour (use a food scale if you have it -- 135g). Add in 1 tablespoon cornstarch, 1-1/2 tsp baking powder, 1/2 teaspoon salt, 1/2 teaspoon cinnamon, 1/4 teaspoon nutmeg, and 1/4 teaspoon allspice. Mix until combined.
WET INGREDIENTS: In a separate bowl, whisk together 1 large egg, 1/2 cup Greek yogurt, 1/4 cup melted coconut oil (See Note 2), 1/2 teaspoon vanilla extract, and 1/2 cup brown sugar. Whisk together until smooth.
APPLE: Peel and very finely chop the apple. Toss with the lemon juice. Add into the dry ingredients and stir until the apple pieces are coated in the flour (this helps to evenly disperse the apple).
COMBINE: Add dry ingredients on top of wet. Mix with a spatula until ingredients are just combined and no dry streaks of flour remain.
FILL MUFFIN CAVITIES: Spoon batter into a 1/4 cup measuring cup to perfectly fill it to the top. Then scoop that batter out into each muffin cavity. This should yield ~10 muffins.
TOPPING: In another bowl, toss together the flour, quick oats, cinnamon, and brown sugar. Pour in the coconut oil (measure when melted) and mix to combine (crumble with your fingertips if needed). Add a bit more coconut oil if too dry or a bit more flour if too wet. Sprinkle this topping evenly on top of each muffin. Don't press the topping into the muffin; just sprinkle on top.
BAKE: Bake for 20-25 minutes or until a toothpick when inserted in the center of the muffin comes out clean. Remove and allow to cool for 5 minutes before removing the muffins from the tin. Use a spoon or butterknife to trace around the edges and coax the muffins out. Allow muffins to finish cooling on a cooling rack so the bottoms don't get soggy as they cool. Enjoy warm!
STORE: Store completely cooled muffins in an airtight container in the fridge. Warm in the microwave for 10-15 seconds before serving, if desired. Muffins are best enjoyed within 2-3 days.
Notes
Note 1: Feel free to increase the spices if desired. As written, the spices yield a very subtle flavor.Note 2: Measure the coconut oil in its melted form. Do not add hot or warm coconut oil to the wet ingredients; wait until it's a room temperature so it doesn't clump.