Preheat oven to 350°. Line two standard 12-cup muffin pans with paper liners.
Beat together eggs, lemon juice, ⅓ cup honey, and ⅔ cup oil in a large bowl.
In a separate bowl, whisk oat flour, almond flour, flaxseed meal, baking powder, baking soda, cinnamon, nutmeg, and salt until smooth.
Add to the egg mixture ½ cup at a time and whisk to combine. Mix in the apple, ½ cup of walnuts, and ½ cup of oats. Pour batter into prepared muffin cups, filling each just about to the very top.
Mix remaining walnuts, oats, 2 Tbsp. coconut sugar, and 1 Tbsp. oil in a small bowl. Sprinkle oat topping over each muffin.
Bake muffins until tops are golden brown, and a tester inserted into the centers comes out clean, 24–28 minutes.