Preheat the oven to 190°.
Place paper liners in the cups of a muffin tray.
Sift together the flour, baking powder, and salt, and set aside.
In a large bowl, lightly whisk the eggs, sugar, and melted (not too hot) butter together.
Whisk in the milk and lemon zest, then gently fold in the apple cubes and blueberries.
Add the sifted dry ingredients and gently fold the batter together until just combined – it’s okay if it’s a bit lumpy.
Spoon the batter into the paper liners, filling them to the brim.
Spoon a generous amount of crumble topping in small mounds on top of the batter, and garnish with the extra blueberries.
Bake the muffins for 30-35 minutes, or until a skewer inserted in the center comes out clean.
Remove the finished muffins from the oven and let them cool slightly before taking them out of the muffin tray.
Serve warm.