To make the wheat grain pudding, pour boiling water over the wheat berries to cover and leave overnight to soak.
Preheat the oven to 200C/180C Fan/Gas 6.
To make the celeriac, place the hay on a baking tray and toast in the oven for 5–10 minutes. Set aside to cool then moisten with a little water.
Wrap the celeriac in an even layer of the hay, place on a baking tray and cover tightly with tin foil. Bake for 20 minutes or until tender then set aside to cool, still wrapped.
To make the hay butter, finely chop 15g/½oz of the hay that was used to bake the celeriac and pour the melted butter over to infuse in a bowl. Strain the mixture through a sieve and place the butter in the fridge to chill, discarding the hay.
To make the chicken and yeast crumb, place the chicken skin between two flat baking sheets and cook in the oven for 5–10 minutes. Remove from the oven, cool and roughly chop the skin.
Toast the yeast flakes in a dry frying pan over a medium heat until golden-brown. Allow to cool then mix with the chicken skin and set aside.
To make the whey sauce, simmer the whey over a medium heat until reduced by two thirds. Add the cream and gradually whisk in the butter. Pass through a sieve, season with salt and pepper and add the verjus. Keep warm.
To finish the wheat grain pudding, drain the wheat berries and cook in a saucepan with the stock until tender. Add the cheese, lemon zest and parsley, stir through and keep warm until ready to serve.
To finish the celeriac, remove it from the hay, heat the hay infused butter in a frying pan and brown the celeriac on all sides until golden-brown. Brush each celeriac with reduced chicken stock and roll in the chicken and yeast crumb.
Fry the trumpet mushrooms and celeriac roots in a little butter, season with salt and pepper and drain.
To serve, place some of the wheat grain pudding on each plate and place a celeriac alongside, spoon over some whey sauce and scatter with the mushrooms, celeriac roots and watercress shoots.