Method

Making the homemade mayonnaise:
First separate the egg yolks from the egg whites.
Then whisk the Dijon mustard and yolks together. You will need half of your oil at this stage. Add a drizzle of oil and whisk well to form a base. Then continue to add the rest of the oil, little by little, whisking constantly until they combine and start to thicken and become glossy.
Next, add one tablespoon of the white wine vinegar (you will add the rest later). This will lighten the mixture and make it go paler. You can add the rest of the oil which will help the mixture thicken, mix well.
To flavour, add the lemon juice and the rest of the white wine vinegar. You can add chopped herbs for a herby mayonnaise or garlic for an aioli.
Place in a sterilised a jar and refrigerate for up to 7 days.

Making your sandwich:
First, boil the eggs. Fill a small pot around halfway with cold water and bring the water to the boil (alternatively, you can boil in a kettle). Place the eggs carefully in the saucepan, being careful not to splash yourself, leave to boil for 9-10 minutes for the perfect hard-boiled egg.
Once cooked, plunge the eggs into a bowl of iced water or allow to cool slightly before peeling them. Placed the peeled eggs in a bowl and use a fork to mash up the egg.
Next, mix the mayonnaise in with the eggs. Add the cress and a sprinkling of finely chopped chives, and some seasoning to taste.
Finally, assemble the sandwich together on lightly buttered sourdough bread. Add the egg mayonnaise filling generously and top with watercress. Sandwich with the other slice of sourdough and enjoy!