Preheat a waffle iron according to manufacturer's instructions.
Mix walnut flour, almond flour, baking powder, vanilla extract, cinnamon, nutmeg, and stevia together in a large bowl.
Mix melted butter and applesauce together in a measuring cup, adding only enough applesauce so mixture is 1/2 cup.
Beat egg whites in the bowl of a stand mixer until fluffy and stiff peaks form. Remove 2/3 of the beaten egg whites and set aside in a bowl.
Place flour mixture and butter-applesauce mixture in the bowl of the stand mixer with the remaining egg whites; beat on low using the stirring paddle until batter is thoroughly combined. The mixture will be dry. Fold reserved egg whites into batter until incorporated.
Ladle 1 to 2 scoops batter into the preheated waffle iron; cook according to manufacturer's instructions. Repeat with remaining batter.