Preheat oven to 200°C. Line a muffin tray with liners.
Prepare crumble topping:In a bowl mix flour and salt.Rub in cold butter until crumbly.Add sugar, oats and walnuts.Mix gently and refrigerate until needed. Make muffin batter:In a bowl combine flour, baking powder and cinnamon.Add chopped apples and toss well. In another bowl whisk milk, brown sugar, oil and vanilla.
Pour wet ingredients into dry ingredients and fold gently.Do not overmix.
Divide batter into muffin moulds.
Generously sprinkle crumble topping over each muffin.
Bake for 20–30 minutes until golden and cooked through.
Cool slightly and serve warm.