In a saucepan cover the brisket with water and bring to the boil. Refresh in cold water. In fresh water add the vegetables and bring back to the boil, skimming the top. Simmer for 3 hours until tender.
Allow the brisket to rest in its stock and keep warm. Strain 1 pint of the cooking stock.
In a pan cook the butter and flour to make a roux. Gradually add the stock to make a veloute. Cook out for 5 minutes then add the cream season with pepper.
At very last minute bring to the boil and add, parsley, gherkin and lemon juice.
In a pan cover the celeriac and potatoes in milk and boil, then simmer for 20 minutes until cooked. Drain and mash, adding a touch of the cooking milk and season.
Slice the brisket, place on top of mash and spoon sauce over.