For the gooseberry sauce, heat the butter in a small saucepan, place the gooseberries into the pan, cover and cook over a medium heat for 20-25 minutes until tender.
Remove the lid and mash the gooseberries lightly. Add the cream and sugar to the pan, season with salt and freshly ground black pepper and continue to cook for a few minutes. Remove from the heat, cover and set aside to keep warm.
Meanwhile, for the mackerel, preheat the grill to high.
Brush the flesh side of the mackerel fillets with the wholegrain mustard, brush both sides with olive oil and season with salt and freshly ground black pepper.
Place the mackerel skin-side up on a baking sheet and place under the grill for 4-5 minutes, or until just cooked through. Set aside to keep warm.
For the new potatoes, place the new potatoes in a saucepan of boiling water and boil until tender. Drain, return to the pan and drizzle over the olive oil. Stir to coat the potatoes and season with salt and freshly ground black pepper.
For the salad, place the salad leaves into a bowl, drizzle over the olive oil and season with salt and freshly ground black pepper. Stir well to dress the salad.
To serve, place the mackerel fillets onto serving plates and divide the new potatoes, salad and gooseberry sauce between them.