Preheat oven to 175°C and line a 9x5" loaf tin with parchment paper.
Sift and mix dry ingredients (flour, baking powder, baking soda, salt, ground cinnamon and ground nutmeg) and set aside.
Wash, peel, core and finely dice the apple. Set aside.
Place granulated and brown sugar in a large mixing bowl of a stand mixer fitted with the paddle attachment. Add vegetable oil and mix on medium speed until combined.
Add eggs, one by one, and mix well (1-2 minutes) after each addition.
Add dry ingredients in 2 additions, alternating with milk and mix until just combined. Do not overmix.
Fold in apples and coarsely crushed nuts into cake batter using a spatula
Transfer batter to loaf tin and top with crumble topping.To make the crumble topping: Melt butter and stir in brown sugar and cinnamon. Add flour slowly and stir continuously until a crumbly mixture forms. Sprinkle evenly over cake batter. Bake in a preheated oven for 45-55 minutes or until a skewer inserted into the center comes out clean.
Cool in loaf tin for 15 minutes, then transfer to a wire rack to cool completely.