Heat 1 tbsp oil and gently cook i large sliced leek, 2 chopped celery stalks and 1 chopped carrot for about 5 minutes, stirring frequently until softened. A 2 chopped garlic cloves and cook for 1 minute.
Rinse 250g (9oz) lentils and add to the pan. Add 750ml (1¼pt) water and crumble in one stock cube. Bring to the boil, then reduce the heat and simmer for 20 minutes or until the lentils are just tender.
Meanwhile, cook 450g (1lb) potatoes and 450g (1lb) celeriac, peeled and cut into chunks in lightly salted boiling water for 10 to 12 minutes or until just tender.
When the lentils are just tender, stir in 150g (5oz) finely chopped mushrooms, 4 tbsp mushroom ketchup or brown fruity sauce, and 1 tsp each of thyme, rosemary and oregano. Cook for another 15 minutes, stirring occasionally. Preheat the oven to 200℃ (180℃ fan)/400°F/gas mark 6.
Mix 1 tbsp cornflour with a little water to a smooth paste and stir in. Cook for another 2 to 3 minutes. Season to taste, then spoon into a large ovenproof dish.
When the potatoes are cooked, drain well. Add 50g (2oz) butter and 4tbsp milk and mash well. Carefully spoon the mash over the lentil mixture.
Bake in the centre of the oven for about 20 minutes until the top is crisp and golden. Allow to stand for 5 minutes before serving
Nutrition information is calculated usingan online nutrition database - is approximate and is meant as a guideline only. (It does not include seasoning with salt and pepper.)The vegetable and lentil bake freezes well. Freeze before baking for up to 6 months. Defrost fully before baking for 40-45 minutes or until the filling is piping hot and the top is golden.