Peel the potatoes, cut them into thick slices and cook in boiling salted water for 8-10 minutes until the potatoes feel soft when tested with a knife.
Peel and slice the onion and fresh garlic. Wash and chop the celery. Wash and slice the mushrooms.
Drain the potatoes when cooked, then mash with a potato masher or fork and wooden spoon. Beat in the butter and grated cheese and season with salt and pepper. Leave in the pan with the lid on.
Heat the oil in a saucepan over a medium heat and fry the onion, celery, and garlic gently for 4-5 minutes until soft. Add the mushrooms and continue to cook gently for 3-4 minutes, stirring occasionally.
Mix the flour or cornflour to a smooth paste with 1tbsp of the milk, stir in the rest of the milk and pour it all into the mushroom mixture, stirring over a low heat until the sauce thickens. Add the nuts, herbs, and washed, scissor-snipped parsley, lemon juice, salt and pepper and simmer for a further 2-3 minutes, stirring gently.
Put the vegetable mixture into an individual pie dish. Top with the cheesy potato and fork the top decoratively. Garnish with the tomato slices and brown under a hot grill until golden brown.