Preheat oven to 200ûF (100°C). In a large mixing bowl, whisk together unbleached and spelt flours, walnuts, sugar, baking powder, baking soda and salt. Set aside.
In separate bowl, whisk together milk, butter, egg yolks and lemon zest.
In another bowl, beat egg whites on medium speed until soft peaks form.
Pour milk mixture over dry ingredients and stir until just combined (batter will be very lumpy). Gently fold in grated apple. Fold in egg whites until just incorporated (do not overmix).
Pour 1/4 cup (65 mL) of the batter onto a lightly greased griddle or skillet over medium heat. Place an apple slice on top of each pancake and cook until top is speckled with bubbles and underside is golden brown, about 3 to 5 minutes. Turn with spatula and cook until apple is golden and tender and pancake is cooked through, about 3 minutes. (Note: If the pancakes are getting too dark before the inside is cooked through, reduce heat and cook for a longer time; the juice from the apple may slightly increase cooking time).
Transfer apple-walnut pancakes to a foil-lined baking sheet and keep warm in preheated oven. Continue cooking pancakes until all batter has been used and serve immediately with maple syrup.