Preheat your oven to Gas Mark 6 (400℉) and place your muffin tins on a baking tray. If using muffin tin liners, place in now, otherwise spray your muffin trays with a light coating of olive oil, or spread with melted butter.
In a large bowl, add all of your muffin ingredients and mix until fully incorporated. Place aside.
(optional) In a smaller bowl, place all your crumble mixture in a bowl and lightly mix until just incorporated. You are wanting large clumps to remain, so be gentle and don't over mix.
Spoon in your muffin mixture about ¾ of the way full. Once you have filled all of your muffins, sprinkle your crumble mixture on top, completely coating the surface.
Place in your oven for 22-25 minutes, until just turning golden. Release the muffins by using a knife to gently pull apart the edges if needed, and place on a baking tray to cool.
Allow to cool to room temperature, and store in an airtight container.