In a medium bowl, whisk together both of the flours, baking soda, cinnamon and salt. Set aside. In the bowl of an electric mixer fitted with the paddle attachment, combine the brown sugar and oil and bet on the medium-low setting. Add the apple sauce and yogurt and continue to beat until fully combined, about 1 minute. Mixture will be gritty and thin. Add the eggs, one at a time, beating well after each addition. Mix in the vanilla. With a spatula, manually fold in dry ingredients into the wet ingredients until just combined, do not overmix. Fold in the chopped apples and walnuts. Divide the muffin batter equally into the muffin cups. Sprinkle the remaining oats on top. Bake for 20 - 23 minute so or until a toothpick inserted in the center comes out clean or with just a few crumbs.