First of all, put the egg yolks into the basin, add the crushed garlic, mustard powder, salt, and black pepper. Mix all of these together well. Then, holding the Stark rapeseed oil in a jug in one hand and an electric hand whisk in the other, add a little of the oil to the egg mixture at a time, and whisk this in.
Boil potatoes until cooked and mix with chopped red onion, spring onion, chives, capers and gherkins. Bind in crème fraiche and correct seasoning – maybe add a little more lemon juice.
For the fish topping: Mix some crème fraiche with grated Arran mustard cheddar, chopped chives, and spring onion and top fish with a generous dollop.