Start by making your pumpernickel toasts. Preheat your oven to 170C. Finely slice the bread, lay it on a baking tray in a single layer and drizzle with a little oil. Toast for approx 6 minutes, turning halfway through, until golden brown.
Once cool, portion each piece of toast into 2 or 3 bitesized pieces.
To make your aioli, put the egg yolk, dijon, lemon juice, garlic and a good pinch of salt into a bowl and combine.
Using a whisk, gradually add the sunflower oil until it is all incorporated and you have a thick, glossy sauce.
Strain the mussels from their marinade and then add the marinade to your aioli. Taste the aioli to check the seasoning and add more salt and / or lemon juice, as necessary.
To assemble to toasts, place half a teaspoon of aioli onto each piece of toast, top with a mussel and finish with a sprig of dill.
Serve immediately to your guests along with a glass of something sparkling.