Julienne the white cabbage and the 2 carrots using a mandoline, so you have fine, even, strips. Place in a large bowl.
Carefully grate (shred) the quarter of an onion using a grater and add the onion to the bowl.
Add the cup (240g) of mayonnaise, 3 tbsp sour cream, 1 tbsp white wine vinegar, 1/2 tsp salt, 2 tsp sugar, 1/8 tsp mustard powder and the pinch of white pepper.
Stir everything together until thoroughly combined.
Serve immediately, or cover and refrigerate until needed (up to 2-3 days).
Adjusting the quantity of coleslaw
You can increase or decrease the amount of coleslaw by adjusting the ingredients proportionately. If you're just looking to make a small amount of dressing - for example you want to make one cup of coleslaw, use: A matchbox-sized piece of cabbage, 1/4 of a carrot, a tsp of grated onion, 2 tbsp mayo, 1.5 tsp sour cream, a few drops of white wine vinegar, a pinch of salt, 1/4 tsp sugar and a tiny pinch each of mustard powder and white pepper.