Make the Chive-Infused Oil
Snip the chives directly into a blender bowl.
Add 100 ml / 0. 5 cup of the sunflower oil and blend for 30 seconds until you have a vibrant green oil.
Scrape the chive oil into a separate bowl and set it aside to infuse. The longer it sits, the more flavourful it will become.
Thoroughly wash and dry the blender bowl. This is a crucial step, as any water from the chives can cause the mayonnaise to split later.
Create the Mayonnaise
In the clean blender bowl, combine the room-temperature egg yolks and the white wine vinegar.
With the blender running on a low speed, begin to drizzle in the remaining 150 ml / 0. 5 cup of plain sunflower oil very slowly until the mixture is thick and emulsified.
Strain the chive oil through a fine-mesh sieve into a jug, discarding the solids.
With the blender on its highest speed, carefully and slowly drizzle in the strained chive oil. As soon as it's fully incorporated, switch the blender off immediately to prevent it from splitting.
Season the finished mayonnaise with salt and pepper, then transfer to a bowl and chill in the fridge.
Assemble the Sandwiches
Place the 8 eggs in a saucepan and cover with cold water.
Bring to a boil and cook for 12 minutes for a perfect hard-boiled centre.
Drain the hot water. Gently rattle the eggs in the pan to crack the shells, then cover with cold water and let them sit for 5-10 minutes to cool completely.
Peel the cooled eggs and roughly chop them.
Add the chopped eggs to the chilled chive mayonnaise and gently mix to combine. Have a taste – it's irresistible!
Slice your bread, generously spread the egg mayonnaise mixture, top with a handful of fresh watercress, and place the final slice of bread on top.
Slice and serve immediately.
After boiling the eggs, crack their shells and let them cool completely in a bowl of cold water. The water seeps into the cracks, making them significantly easier to peel.