Mix the oats, sugar, spice, lemon zest, salt, flour and baking powder in a large bowl.
Pour the milk into a second bowl then melt the butter and add to milk with the cream of tartare, vanilla and beaten egg.
When the oven is ready stir the liquid mix into the dry ingredients. Once the liquid is added to the flour you need to work quickly to avoid loosing aeration and making the muffins heavy.
Fold in the grated apple, pour into muffin cases, then sprinkle with a few of the extra oats, and bake for about 20 minutes.