Put the mustard, dill, sugar and vinegar in a jar and put the lid on top and shake well. Now add the oil, a little at a time shaking to emulsify each time. Add some more oil if it gets too thick add a tablespoon of water and shake again, season to taste with salt and freshly ground black pepper.
Asparagus and Potatoes:
Allow about 4/5 potatoes and about 6 spears of Asparagus per person cook the potatoes and asparagus in separate pans of salted boiling water,don't over cook the asparagus. Once cooked reserve and keep warm. The potatoes will take 15 /20 min the asparagus about 3/4 mins.
If you can get a fresh Horseradish Root, peel it and grate about 3 tablespoons of it into a bowl add 3 tablespoons of Mayonnaise and 2 tablespoons of double cream mix all together and create a reasonably firm sauce. If you are unable to source fresh Horseradish [which is very hot!] use the very best prepared horseradish you can get and add cream to it.
Now Grill the Mackerel brushed with oil till just cooked,
Cut the warm potatoes in half and the asparagus spears. Toss them together in a bowl with the dill vinaigrette and put onto the serving plate with the mackerel on top. Dot the rim of the plate with the horseradish dressing and garnish with fresh dill.