Sift the flour into a mixing bowl, add a pinch of salt and whisk in the egg yolk. Add the milk and water and whisk like crazy until all the lumps are out of the batter. Set aside for 30 minutes.
Peel, core and thinly slice the apples and place the slices in a casserole or au gratin dish. Grate the lemon zest over the apple slices then add the mixed spice, brown sugar and walnut oil. Stir to combine.
Place the casserole in the middle of the oven and set oven to 400Ëš Fahrenheit. When the oven comes up to temperature, take the casserole out, give the apple mixture a stir and pour the batter over the hot apple mixture. Return casserole to oven and bake 20 to 25 minutes, or until batter is golden brown and cooked through. Serve warm.