Preheat the oven to 180 degrees Celsius/ 350 degrees Fahrenheit. Line a 12- muffin baking tray with muffin paper cases.

Prepare apples: Weigh 250 g/ 8.8 oz apples and peel them. Halve and core them. Cut 12 thin apple slices, place them in a bowl with 1 tablespoon of lemon juice and toss them with the liquid. Set aside; you will need them to top the muffins. The lemon juice will prevent the apples from browning too quickly.Chop the rest of the apples, place them in another bowl, toss them with the remaining lemon juice, and set aside.

Cinnamon sugar: In a small bowl, mix 2 tablespoons white granulated sugar and 2 teaspoons of cinnamon. Set aside.

Dry ingredients: Mix 250 g flour, 3 teaspoon baking powder, 150 g brown sugar, a pinch of salt, and the remaining 2 teaspoons of cinnamon. Set aside.

Wet ingredients: In another bowl, large enough to hold all the batter, whisk the 2 eggs, 125 ml oil, 150 ml milk, and 1 teaspoon vanilla extract very well. Add the flour mixture, one tablespoon at a time, to the egg mixture and incorporate. Add the cubed apples. Don't overmix.

Topping: Place the batter in the prepared muffin tin. Place one slice of the remaining apples on top of each muffin and press it lightly into the batter. Sprinkle each muffin with the sugar-cinnamon topping mixture.

Bake the muffins into the preheated oven for about 20 - 23 minutes or until golden. A toothpick inserted in the middle of the muffin should come out clean. Unless you hit an apple piece which will cause the toothpick to be wet, so pay attention that you only hit the batter. Do not overbake.

Cool: Leave the muffins in the muffin pan for about 5 minutes, take them out, and let them cool completely on a wire rack.