Mix 1 cup milk with 1 teaspoon lemon juice in a medium bowl. Add 1 ½ cups traditional rolled oats, and leave to hydrate while preparing the rest.
Turn on the oven at 350°F (180ºC). Line 12 muffin cups with paper liners. Or butter and flour them if not using paper cups. Reserve.
Sift 1 ½ cups all-purpose flour, ½ teaspoon salt, ½ teaspoon baking soda, 1 ½ teaspoons baking powder, 1 teaspoon ground cinnamon and a pinch of allspice or nutmeg (if using) into a large bowl.
Add ½ cup light brown sugar to the oat mixture and stir to combine.
Add ½ cup oil, 1 egg and ½ teaspoon vanilla extract to the oats, or stir to mix. Integrate everything well without beating.
Add the wet ingredients to the dry ingredients, then 1 cup chopped baking apple and stir with a spatula until just combined. Don’t be tempted to beat it or overmix it as it will result in tougher and less fluffy muffins.
Fill the muffin tins or paper cups no more than ¾ full, dividing the mixture as evenly as possible.
Sprinkle the reserved oat topping on top of the muffins.
Bake for 20-25 minutes or until a tester comes out dry. Let them cool on a wire rack.
Store muffins that are left in an airtight container in the refrigerator, and reheat briefly before eating. Or in the freezer for a month.