Pre-heat the oven to 180 degrees centigrade.
In a large ovenproof dish heat the oil and add all the spices.
Cook for 30 seconds.
Add the onion, celery, red pepper, sweet potato, potatoes, garlic, ginger and any other vegetables you are using.
Cook until softened but not browned.
Add the tin of chopped tomatoes and stock.
Stir together and bring to a simmer.
Add the lentils and bring to a simmer again.
Put the lid on and put the dish in the oven.
Cook until the lentils are completely soft and the sweet potato has turned to mush.
Season to taste.
Serve with couscous.