Rub the celeriac with the unsalted butter and half a tablespoon of salt and wrap in aluminium foil. Roast in the oven for 45 minutes, or until cooked all the way through.
Meanwhile, crush the juniper berries with a mortal or the back of a knife. Put in a saucepan with the white wine vinegar, sugar and a teaspoon of salt and bring to a boil. Whisk until the sugar has dissolved. Add 500ml/18fl oz cold water.
Put the beetroots in the pickling liquid and set aside for 30 minutes.
To make the croutons, melt 2 tablespoons of butter in a frying pan over a medium heat.
Fry the cubes of bread for 5–10 minutes, or until golden brown and crispy. Remove with a slotted spoon and drain on kitchen paper. Season with salt and pepper.
In the same pan, make the brown butter sauce. Add the remaining 2 tablespoons of butter and cook until golden brown. Add the sherry vinegar and season with salt and pepper.
Mix together the pickled beetroot slices, peashoots and celery leaves. Dress with a little of the pickling liquid.
To serve, cut the celeriac into quarters. Dive between four plates and top with the croutons, hazelnuts, salad and a drizzle brown butter sauce.