Cook fish with tarragon, salt and pepper and flake it.
Mash potatoes with a little milk and butter.
Mix potato and fish together.
Use pastry cutter to shape fish cakes by putting it inside the pastry cutter and use a spoon and press down a layer of potato/fish then a layer of mushed peas and then another layer of potato/fish.
Sprinkle sesame seeds on top and remove cutter.
Repeat to make 4 fish cakes.
For the mayonnaise: Separate egg yolk and white, put egg yolk in a small bowl. Whisk egg white for a minute then SLOWLY add olive oil, add crushed garlic, salt and pepper. Mush peas and mix with lemon juice. Whisk pea mixture into mayonnaise.