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Place the eggs in a saucepan and cover with cold water. Bring to the boil then simmer for 6½ minutes. Drain and run cold water over the eggs until they have completely cooled.
To make the mayonnaise, put the egg yolk in a food processor and pulse. Add the mustard, white wine vinegar and a pinch of salt then blend. Add oil a few drops at a time, keeping the processor running and making sure each bit of oil is fully incorporated before adding the next. As the mayo thickens, steadily add the oil in a thin stream until it reaches a consistency that’s thick and holds its shape fairly well. Don’t add too much oil or the mayonnaise will split. Add salt to taste.
Peel the eggs, and mash them with a fork. Mix in 2 tbsp of your mayonnaise, then season to taste with salt and white pepper.
Butter the bread then spread thickly with the egg mixture before topping with chopped watercress. Top with another slice of bread then cut each sandwich into four triangles. You can even remove the crusts!