Preheat the grill to its highest setting.
Soak dulse seaweed in cold water for 10 seconds, drain, and chop finely in a food processor.
Blend the seaweed with softened butter and lemon juice until combined.
Butterfly the mackerel by removing the rib cage and bones using a sharp knife.
Spoon most of the seaweed butter down the center of each mackerel.
Place mackerel in open foil parcels onto a tray and grill for 5-10 minutes.
Simmer shallots and broad beans in 100ml water for 2 minutes.
Add peas and pods to the pan and simmer for 1 minute.
Add asparagus, tarragon, and the remaining seaweed butter, simmering for 2 minutes until tender.
Plate the mackerel and vegetables, drizzling remaining butter over the fish.