Pickled asparagus tonnato

Method

1. Trim off the woody ends of the asparagus (and remember to save them!) then put in a pan with the vinegar, water, sugar, mustard seeds and dried dill. Bring to the boil and simmer for 2 minutes, then take off the heat and set aside to cool in the liquid.
2. To make the sauce, put the tuna and egg yolk in a food processor and whizz together. Then, with the motor still running, slowly drizzle in the olive oil in a thin stream. If the mixture seems a little thick after adding all the oil, add a splash of water – you want it to be a little thicker than double cream, but still loose enough to pour. Taste and season with plenty of lemon juice, salt and pepper.
3. Pour the sauce onto a serving dish, then drain the asparagus and arrange the spears on top. Sprinkle over the red onion and dill sprigs then finish with a drizzle of extra virgin olive oil.