1. Preheat the oven to 350°F (180°C). Lightly grease a 9×5 inches (23×13) loaf pan and line with a strip of baking parchment. Set aside. Melt the butter for the batter and the topping separately and let cool down a bit. Peel, core, and dice the apple finely and set aside. Chop the walnuts and set aside.

2. Add the sugar, yogurt, pumpkin puree, eggs, vanilla extract, and melted butter to a large bowl and mix until well combined. Mix the two flours with baking powder, cinnamon, and salt and add to the large bowl – mix everything until just combined. Add the diced apples and fold in. Pour the batter into the prepared loaf pan and smooth out the top.

3. Add the chopped walnuts, brown sugar, flour, and cinnamon to a small bowl and mix. Add the melted butter and mix everything until well combined and evenly moistened. Sprinkle the mixture over the batter and bake for 55-60 minutes or until a wooden skewer inserted into the bread comes out clean. Take out of the oven and let cool down in the pan for some time, then remove and let cool down completely on a wire rack. When cooled drizzle with some sugar glaze (optional) and serve. Leftover pieces can be frozen.