Put the cooked beans and measurement water in a food processor and blend them to a smooth paste. Cut away the skin from the celeriac, cut the flesh into chunks and cook them in a saucepan of lightly salted boiling water, with the potatoes, for about 15 minutes, until tender.
Meanwhile, pat the salmon fillets dry on kitchen paper and season them with salt and pepper. Heat 15 g (½ oz) of the butter in a frying pan and fry the salmon for 4–5 minutes on each side, until it is cooked through.
Drain the vegetables and return them to the saucepan with the blended beans and a further 15 g (½ oz) of the butter. Using a potato masher, mash the ingredients together until evenly combined. Reheat for 1–2 minutes and season with salt and pepper.
Pile the mash on to warmed serving plates and top with the salmon fillets. Add the remaining butter, herbs and vinegar to the frying pan and heat through until the mixture bubbles. Pour the sauce over the salmon and serve immediately.