For the Yankee sauce, hard boil eggs for 8-10 min. Separate the yolks, mash with a fork and mix with salt and mustard until smooth.
Gradually stir in the oil with a little lemon juice.
Add a finely chopped onion and finely chopped cornichons and season the sauce with salt, a pinch of sugar and some lemon zest. Finely chop egg whites and mix in.
Serve Yankee sauce with fried fish fillets, fish sticks, cold meat and poached eggs respectively.