Poach the chicken by placing it in a small stock pot (approximately 8 litres) and fill with cold water until the chicken is completely covered. Cook for 7-10 minutes depending on the size of the fillet.
Make mayonnaise by combining egg yolk, mustard and 2 tablespoons of white wine vinegar in a food processor. Mix together well and with the motor running slowly drizzle in the olive oil. The mix should emulsify and become thick. (If you add the oil too quickly the mayonnaise will separate). Season with salt, freshly ground pepper and a splash of water just to achieve the correct consistency.
Carefully break up and lightly shred the poached chicken with your fingers and place in a mixing bowl.
Chop the dill and add to the chicken. Trim the spring onions and finely chop, and then add them to the chicken mix along with a little over half the mayonnaise. Season the chicken well with salt and freshly ground black pepper and another tablespoon of white wine vinegar. Mix gently with a spatula to combine, do not overmix. The mixture should be quite wet, add some more mayonnaise if necessary.
Peel and slice a continental cucumber.
To assemble, spread 4 slices of bread with the chicken mix, top with the sliced cucumber followed by the rocket. Season with freshly ground black pepper and top with the other remaining slice of bread and enjoy. For best results do not use sourdough bread, the chicken mix will keep well and can be made a few days in advance. Always check the seasoning before using, and if the flavours have fallen a little flat, add some more white wine vinegar and some salt and pepper.