Place a large frying pan over a medium heat. Add the olive oil and fry the onions, carrot and celery for 7-8 minutes until starting to soften and brown slightly.
2 carrots, 1 onion, 1 tablespoon olive oil, 1 stick celery
Add the garlic, herbs, Marmite, tinned tomatoes and tomato puree. Mix well.
1 clove garlic, 400 g canned tomatoes, 1 tablespoon fresh thyme, 1 teaspoon Marmite, 1 tablespoon tomato puree
Add the lentils and stock. Bring to the boil, cover and simmer gently for 30 minutes. Stir occasionally.
200 g red lentils, 700 ml vegetable stock
Meanwhile, boil the sweet potato cubes for 15 minutes. Drain well and mash, seasoning with the grated nutmeg. Stir in half of the grated cheese.
750 g sweet potatoes, 1 pinch grated nutmeg, 120 g mature cheddar cheese
Pile the red lentil mixture into an overproof dish and top with the sweet potato. Sprinkle with the remaining grated cheese.
If the mixture is freshly cooked and still hot, pop it under a preheated grill to brown the top. If you've prepped the dish before hand and it has been in the fridge, place in a preheated oven at 180 degrees for 30 minutes to heat through thoroughly.