Preheat the oven to 350°F (180°C) mark 4.

Prepare the mushrooms by wiping with a damp cloth and trimming off the ends of their stalks; slice thinly and cook in the oil with the garlic and half the chopped parsley, stirring to cook lightly and evenly; make sure that the moisture they release evaporates. Season with a little salt and pepper; process one third of the mushroom mixture in the blender with the cream, milk, 2 whole eggs and the extra yolks, the Parmesan and a pinch each of salt, pepper and nutmeg. When smoothly blended, transfer the mixture into a bowl and mix in the reserved mushrooms and the rest of the parsley evenly.

Grease a 2 pint (1.2 litre) lidded terrine dish with butter and line the bottom with greased waxed paper. Fill with the mixture, cover with the lid, sealing the join with a luting paste made with 2 tbsp flour mixed with a little cold water; leave the small hole in the lid free for the steam to escape. Alternatively, cover tightly with a sheet of foil in which you have pricked a few small holes. Place in a roasting tin, add sufficient boiling water to come half way up the sides of the terrine and place in the oven. Cook for 50 minutes.

Make the sauce. Peel the celeriac, slice very thinly using a mandoline cutter or food processor; cut these slices into julienne strips. Heat the oil in a wide, fairly deep frying pan with the thyme and stir-fry the celeriac over a moderately high heat until tender but still fairly crisp. Add the peeled, seeded and coarsely chopped tomatoes and the basil leaves cut into thin strips.

Season with salt and pepper and simmer for 2 - 3 minutes while stirring; set aside until the mushroom terrine is cooked, then reheat the sauce just before serving, adding 1 tsp lemon juice. Stir in the butter off the heat. Turn out the hot, cooked terrine and serve with the sauce.