Heat the stock. Wash, dry, peel and dice the celeriac; place in a saucepan and add about half the boiling hot stock, sufficient to cover it. Cover and boil gently until the celeriac is very tender; put through a vegetable mill to puree.

Heat the butter over a low heat in a very large, heavy-bottomed saucepan; stir in the rice flour and cook for 1-2 minutes or until it begins to turn a pale golden brown. Draw aside from the heat and beat in the remaining hot stock.

Return to the heat and bring very slowly to the boil while stirring continuously. Simmer for a further 5 minutes before stirring in the celeriac puree. Reduce the heat to very low.

Beat the egg yolks and cream them together in a small bowl or jug until blended; beat this thickening liaison into the soup, adding a little at a time. Continue to beat for a few minutes, until the soup is very hot, but do not allow to boil or it will curdle.

Draw aside from the heat, stir for 1-2 minutes and season to taste with salt, pepper and a little lemon juice.

Sprinkle with parsley and serve with the croutons, fried in butter until golden brown.