Rinse the bunch of dill, trim off the thickest parts of the stalks and spin-dry them in a salad spinner or a tea towel. Place them in a blender with 300 ml of the oil and blend for a couple of minutes until the oil is warm, green and free of whole pieces of dill. If necessary, pass the oil through a fine-mesh sieve if you want the mayonnaise to be completely smooth. Now mix the herb oil with the rest of the oil.
Place the egg yolks in a large bowl along with the mustard, a little salt, freshly ground pepper and 2 tbsp lemon juice, and whisk well using an electric whisk, or if you have a good balloon whisk to make the mayonnaise. Now whisk the dill oil into the egg yolks a little at a time in a thin stream. Pour slowly enough to ensure the oil is constantly being whisked into the egg yolks. Pause occasionally to allow the mayonnaise to be thoroughly whisked. Continue until all the oil has been added, then whisk thoroughly once more.
Finally, season to taste with salt and, if desired, extra lemon juice; the dill mayonnaise tastes best if it is slightly on the tart side. Transfer the dill mayonnaise to a piping bag.
Refrigerate the dill mayonnaise immediately, where it will keep for 5–6 days.
Cut the lemons in half and remove any seeds.
Heat the barbecue or your grill pan well, then grill the lemon halves, cut-side down, until they have black grill marks, then remove them. Place the bread on the barbecue too and toast it briefly, so it takes on a little colour but doesn’t become hard.
Spread a little dill mayonnaise on the warm bread and arrange the prawns on top, with a few of the tender young salad leaves. Place small dollops of dill mayonnaise between the prawns and salad, and garnish with sprigs of dill. Serve with the grilled lemon halves on the side, on your finest plate.
