Preheat the oven (electric cooker: 190 °C). Butter a 12-piece muffin tin and put aside.
Defrost the berries, then drain well. Peel, quarter, core and grate the apple. Put the berries and apple in a large bowl. Add maple syrup, yoghurt and muesli and mix everything carefully.
Mix the cane sugar with the oil. Add eggs and milk and mix everything well and thoroughly with a whisk. Mix the dry ingredients (wholemeal flour, baking powder, bicarbonate of soda, salt) in a second bowl, then add to the liquid ingredients (whole cane sugar, vegetable oil) and stir with a wooden spoon or rubber spatula only briefly until all ingredients have just mixed.
Add the muesli mixture and fold in only briefly.
Spread the dough evenly on the muffin tin. Bake in the oven for 21 minutes. As a cooking test, prick a muffin with a wooden skewer. If the skewer stays clean, the muffins are ready. Stick a little more dough on them, put them back in the oven and bake for a few minutes longer.