Preheat the oven to 200°C.
Peel the potatoes and sweet potatoes and cut them into even sized chunks.
Cook in a pan of boiling salted water, until a knife slides easily into the potatoes. This should take around 20 minutes.
Meanwhile, make the filling. Spray a large pan with some low calorie cooking spray, add the onions, peppers and carrots and cook over a medium heat for around 4-5 minutes.
Add the ginger, garlic granules, curry powder and Marmite. Mix well, then stir in the tomato puree, lemon juice, frozen peas and stock.
Bring to the boil, add the drained lentils and simmer uncovered for 15 minutes, until the veg is cooked through.
When the potatoes are cooked, drain and return to the pan and mash well. Stir in the turmeric and season with some salt and pepper.
When the veg is cooked, place the mix into a deep oven proof dish.
Spread the mashed potato on top and fluff it up with a fork.
Place in the oven for around 30 minutes or until the topping is golden brown.
Serve with plenty of green vegetables.