For the hake, fillet the hake and reserve the bones. Score the skin with a sharp knife, rub the skin side with olive oil and season with salt and freshly ground black pepper.
Heat a frying pan, place the hake fillets skin side down and pan fry each fillet for 3-4 minutes on each side, or until cooked through. Remove from the heat and keep warm.
For the dill new potatoes, place the potatoes into a pan of boiling water and boil for 15 minutes, or until tender. Drain and then place back in the saucepan.
Lightly crush the potatoes with the back of a fork, add the olive oil, butter, salt and freshly ground black pepper, dill, sherry vinegar and finely chopped red onion. Stir well and then keep warm.
For the Mustard grain sauce, place the reserved fish bones into a frying pan with the olive oil, garlic, chilli flakes, onion and boiling water and cook over a low heat for ten minutes. Sieve and discard the bones but reserve the cooking liquid. Reduce fish stock by 3/4, so that there is a lot of flavour. Reduce the white wine by 3/4 with 2 shallots. Add all together and add cream, bring to boil, sieve and add 1 tsp of mustard grain and mix.
For the pea purée, place the blanched and refreshed peas, milk or cream, seasoning and butter into a small pan and warm through. Blend with a stick blender, add a splash of water if the mixture is too thick and then pass through a sieve.
To serve, spoon the pea purée in dashes across the serving plate, place the potatoes in the middle of the serving plate, top with the Hake fillets and serve some of the sauce on the side of the plate.