For the watercress purée, heat the butter in a pan over a moderate heat. Add the watercress stalks and season well with salt and freshly ground black pepper. Cook for 2-3 minutes, until softened, then add the chicken stock and the majority of the watercress leaves (reserving a small handful of attractive leaves for garnishing). Cook for one minute, until the watercress leaves have just wilted.
Allow the watercress mixture to cool, then transfer to a food processor and blend until smooth. Pass the purée through a fine sieve and keep warm.
For the leeks, bring a pan of salted water to the boil, then add the leeks and cook for one minute. Drain and refresh in iced water.
Dry the leeks thoroughly, then place onto a hot griddle pan. Griddle the leeks for 1-2 minutes on each side, until brown griddle lines cover the leeks. Remove from the heat and keep warm.
For the halibut, heat a non-stick frying pan until moderately hot, then add the vegetable oil.
Season the halibut steaks on both sides with salt, then place into the pan and fry until lightly golden-brown, then gently turn over.
Add the butter, then cover the fish with the butter paper and remove from the heat.
Pour the hot chicken stock over the top of the butter paper and allow to rest for two minutes.
Place the pan back over a moderate heat and cook for 2-3 minutes, or until the fish is just cooked through. Remove from the heat and set aside to rest.
For the coddled eggs, place the butter into a separate pan over a high heat and cook until it starts to turn brown. Add a good pinch of table salt and the lemon juice.
Gently break the tops off the quails' eggs.
Heat a pan of water until simmering, then place small egg cobbling moulds, or ramekins, into the simmering water. Fill each with ½ tsp of the browned butter.
Add a quail's egg into each of the moulds and allow the water to simmer gently until the eggs have just set. Remove the moulds from the water and, using a teaspoon, gently scoop out the eggs onto kitchen paper. Season, to taste, with salt and freshly ground black pepper.
To serve, spoon the watercress purée onto the centre of each plate. Top with a piece of halibut, and three leeks. Place a coddled egg on top of each and scatter the watercress leaves around. Place two cobbled eggs either side of the halibut steaks.
Sprinkle the chopped cobnuts over and season well with salt and freshly ground black pepper and finish with a drizzle of olive oil.