In a large bowl, whisk together the Greek yogurt, Dijon mustard, salt, and pepper until smooth. The mixture should be a pale yellow colour and have a thick, creamy texture.

Add the diced eggs, chopped fresh dill, and chopped fresh chives to the bowl and stir until well combined. The mixture should be a vibrant green colour and have a fresh, herby aroma.

Taste and adjust the seasoning as needed. You should be able to hear the sound of the spoon scraping against the side of the bowl as you stir.

Cover the bowl with plastic wrap and refrigerate for at least 30 minutes to allow the flavours to meld. The mixture should be chilled and firm to the touch.

Just before serving, stir in the olive oil and spoon the egg salad onto the wholemeal bread rolls. The bread should be soft and lightly toasted, with a gentle crunch as you bite into it.

Garnish with lettuce, tomato, and cucumber slices and serve immediately. The salad should be fresh and crunchy, with a satisfying crunch as you bite into it.