Preheat the oven to 180C (355F) and line a muffin tin with 12 muffin liners.
In a bowl, combine the flour, salt, baking powder, and cinnamon.
In another bowl, add the white and brown sugar, milk, butter, and oil. Whisk until homogenous in consistency.
Add the dry ingredients to the wet ones and mix until no dry ingredients are left.
Fold in the cubed apples and spread the batter between the muffin liners so they’re ⅔ full.
Sprinkle crushed walnuts on top and bake for around 27 minutes or until a toothpick inserted in the center of a muffin comes out clean.
Let cool down in the muffin pan for 10 minutes, then place onto a cooling rack to cool down completely.