Drain the potatoes, add 25g (1 oz) butter and 60ml (4tbsp) milk, the mash until creamy.
Season well.
Melt the remaining butter in a large saucepan.
Add the celery and onion and cook gently until the onion is transparent.
Add the mushrooms and garlic, and cook, stirring occasionally, for 5 minutes.
Blend the arrowroot with a little milk, stir in the remaining milk and stir into the mushrooms.
Add the parsley, thyme and lemon juice, and season to taste.
Simmer gently for 5 minutes.
Turn the mixture into an ovenproof serving dish.
Top with the mashed potato and place under a hot grill until heated through and golden, or place in the oven at 190°C (375°F/ Mark 5) for about 20 minutes.