To make the gooseberry purée, put the gooseberries, sugar and a squeeze of lemon juice in a pan and place over a medium heat. Cover and cook, shaking the pan every minute or two to break down the fruit slightly (you can see they are ready when the skin starts to break down). Transfer to a food processor and blend until smooth. You may need to adjust with a little sugar to taste. Sieve and chill. If you are using tinned gooseberries, omit the sugar and lemon juice, just blend until smooth. Either way, sieve the purée.
Put the oil for the nori oil in a small bowl in the freezer; it needs to be in there for an hour before you make the flavoured oil.
For the marinade, first take 4 small sprigs of dill from the pack – also set aside a few small sprigs to use as a garnish and the rest of the dill will be used in the nori oil (you will only need the leaves). Chop the four dill sprigs and mix with the other marinade ingredients, apart from the mackerel, in a bowl. Sprinkle half the marinade evenly over the surface of a baking tray. Lay the mackerel, skin side down, over the marinade then sprinkle the remaining marinade over the fillets and cover with clingfilm, refrigerate for one hour.
Meanwhile, make the horseradish yogurt by mixing all the ingredients in a bowl; season to taste. To make the nori oil, bring a pot of water to the boil with a good pinch of salt, add the parsley and dill leaves, and boil for 3 minutes. Drain and refresh the herbs in ice water. Remove the herbs and squeeze out as much excess water as possible. Place the nori in a food processor and blend until finely chopped, then add the blanched herbs and slowly add the oil straight from the freezer, blend for 2 minutes until the mix is smooth; season. You won't need all the nori oil for this recipe but it keeps well and is nice drizzled over griddled fish.
Once the hour is up, rinse the fish in cold water and pat dry. Blanch the asparagus in boiling salted water, drain and refesh in cold water. To cook the mackerel, shake off as much of the marinade as possible (the rest of the marinade can be discarded). Take a non-stick pan and place over a high heat with a drizzle of olive oil. Place the fillets skin side down in the hot oil and sear the fish for 30 seconds on each side. You still want the fish to be pink in the centre. Break the fillets into bite-sized pieces and toss the asparagus and fennel in 2 tablespoons of the nori dressing.
To serve, place a mound of the dressed vegetables in the middle of six dishes (I like to use washed sardine tins). Arrange the mackerel and cucumber among the vegetables. Dollop the gooseberry purée and horseradish yogurt in the spaces and finish with small drizzles of the nori dressing. Garnish with herbs, dill, edible flowers or micro chives.