Prepare the crumble by mixing all its ingredients until you get a sandy mixture. Squeeze the dough with your hands to form large chunks of crumble (this is to make the crumble tighter, crunchier and to help it stick to the muffin batter).

Reserve in the fridge.

Preheat the oven to 355°F (180°C). Grease and dust a muffin tin or skip this step by using muffin liners. Reserve.

In a medium bowl, mix the milk with the vinegar and beat for a couple of seconds. Let it rest for 8 minutes.

In a large bowl combine the flour, baking powder, baking soda, and salt. Reserve.

To the previous milk, add the sugar, oil, and vanilla and beat until the sugar dissolves.

Gradually add the milk mixture to the dry ingredients, whisking until no large lumps of flour are visible. NOTE: Do not over-mix as it can affect the texture of the muffins. Fold in the mixed berries and lemon zest.

In equal portions, pour the batter into the previously prepared pan, filling about ¾ of its capacity (will make about 8 large muffins).

Add crumble on top of the muffins (make sure to cover the surface completely) and press lightly to adhere to the batter.

Bake for 25-28 minutes or until a toothpick or knife inserted into the center of a muffin comes out clean.

Take out of the oven and let it cool down.

Store in an airtight container in a cool place.