Put some water in a saucepan and gently add 2 large eggs (50 g each w/o shell). Make sure the eggs are completely covered by the water. Bring the water to a boil over medium heat. Meanwhile, rotate the eggs with a spoon once in a while to make sure the egg yolks stay in the center. Once the water boils, lower the heat and simmer for 12 minutes. Discard the water and run the eggs under cold water or transfer to iced water to let cool completely before peeling.
Peel the hard-boiled eggs and mash them with the backside of a fork in a medium-sized bowl.
Finely mince ¼ onion. We call this cutting technique mijingiri in Japanese.
The finer the mince, the better the texture is for this recipe. Soak the minced onions in cold water for 10 minutes. This helps to remove their pungency and soften their flavor.
Cut 4 sweet gherkins into small pieces.
Finely chop 3 sprigs parsley.
To the bowl with the mashed boiled eggs, add the minced onion and chopped pickles.
Add the chopped parsley, ¼ tsp Diamond Crystal kosher salt, freshly ground black pepper, and 4 Tbsp Japanese Kewpie mayonnaise.
Mix it all together. Add 1 Tbsp lemon juice squeezed from ½ lemon. Mix again.
Combine 1 tsp Japanese karashi hot mustard with ½ Tbsp hot water and mix well. Add as much of the karashi mustard to the tartar sauce as you like, mixing in a small portion at a time. Cover and set aside in the refrigerator until ready to use.
