For the steamed sole, place the sole fillet strips onto a work surface and place the baby leeks across the fish. Season with salt and freshly ground black pepper and roll up the baby leeks in the sole fillet strips. Carefully place the fish rolls into a steamer over a pan of boiling water and steam for 5-7 minutes, or until cooked through.
For the mustard mash, boil the new potato halves until tender and then drain. Return to the pan, add the mustard and cream and mash well with a potato masher.
Place the steamed sole onto a serving plate with the mashed potatoes. Garnish with chopped dill and parsley and serve immediately.