Vegetable and lentil bake

Name: Vegetable and lentil bake
Ingredients: 1 tbsp rapeseed oil; 1 large leek (washed and sliced); 2 sticks celery (chopped); 1 large carrot (chopped); 2 clove garlic (chopped); 250 g brown or green lentils; 750 ml water; 1 vegetable stock cube; 150 g chestnut mushrooms (finely chopped); 2 tbsp mushroom ketchup or brown fruity sauce; 1 tsp each dried thyme, rosemary and oregano; 1 tbsp cornflour; salt and freshly ground black pepper; 450 g floury potatoes (peeled and cut into 2.5cm/1inch chunks); 450 g celeriac (peeled and cut into 2.5cm/1inch chunks); 50 g butter; 4 tbsp milk