Put your potatoes on to cook, then when ready mash with a little of the cooking water plus salt and pepper. Rinse and cook your green lentils according to the packet instructions.
Fry an onion and some garlic in a little olive oil, then add some chopped celery and cook a little longer. Add some chopped parsley too, if you like it.
Make some stock in a jug with a stock cube and some tomato purée. Add to the vegetables along with the cooked green lentils. Season with some chilli pepper according to taste.
Tip the mixture into a large casserole dish and top with the mashed potato. Sprinkle some nutritional yeast on top and pop in the oven 180C for about 20 minutes. The dish is ready when the topping is lightly toasted.