Prep the fruit: Remove the ends of the rhubarb stems and cut them into a manageable size then slice them on the length to make thinner sticks. Dice them into approximately half inch pieces. Grate the apples and gently squeeze out some of the juice, just enough to prevent the batter from becoming too wet. Add the diced rhubarb and grated apple to a bowl.

Mix the wet ingredients: In a bowl, whisk together the egg, oil, vanilla extract, sugar, cinnamon, salt, a little lemon zest and sour cream until smooth and well combined. Add the wet mixture to the grated apple and diced rhubarb. Stir gently to combine everything evenly.

Make the batter: In a separate bowl, mix the flour, baking powder and baking soda. Fold the dry ingredients into the wet mixture until just combined. Be careful not to overmix.

Fill the cases and bake: Spoon a heaped tablespoon of batter into 12 cases in a prepared muffin tin. Fill all the cases, then distribute any leftover batter between them until they are roughly even. Sprinkle the tops with a little demerara sugar, then bake for 20 to 25 minutes at 190°C or 375°F, depending on your oven type. Check with a toothpick inserted into the center of a muffin. It should come out clean.

Cool and enjoy: Let the muffins cool in the tin for a minute before moving them to a wire rack to cool.