Preheat the oven

• Preheat the oven to 190 °C.
• Line a muffin pan with paper liners.

Make the blueberry jam

• Combine the blueberries, sugar and lemon juice in a small saucepan and cook over medium heat until the berries begin to burst, lightly mashing some of them.
• Mix the cornstarch and water together, stir into the blueberries and cook until thick and jam-like.
• Remove from the heat and let cool completely.

Make the crumble topping

• Combine the flour and sugar in a small bowl.
• Cut in the cold butter until crumbly with some larger chunks, adding more flour if needed.
• Refrigerate while you prepare the batter.

Make the muffin batter

• Rub the lemon zest into the sugar in a large bowl until fragrant.
• Whisk in the egg, vanilla, lemon juice, oil and milk until smooth.
• Add the flour, baking powder and salt and mix until just combined.

Assemble the muffins

• Fill each muffin liner with batter.
• Add a spoonful of blueberry jam and gently swirl it in.
• Generously sprinkle with the crumble topping.

Bake

• Bake for 18–22 minutes, until the tops are lightly golden and a toothpick inserted into the center comes out clean.

Cool

• Let cool in the pan for 10 minutes before transferring to a wire rack.