Step 1

Carefully lower the eggs into a pan of gently simmering water and cook for 7 minutes. Drain, rinse in cold water, then leave in a bowl of cold water until cool (about 5 minutes).

Step 2

Meanwhile, put the egg yolk, mustard, vinegar and a pinch of salt in a tall jug. Whizz with a stick blender, then gradually pour in the oils, without wiggling the blender around too much, until you have a smooth mayonnaise. (You can also do this in a jug blender.) Transfer to a large bowl, then use kitchen scissors to snip in the chives; season.

Step 3

Peel the eggs, then cut each one into 12 chunks; stir into the mayonnaise. Butter the bread, then top 4 slices with the egg mayonnaise and watercress. Sandwich together with the other slices of bread.