In a large saucepan saute the onion and garlic until soft and tender.
Add the chopped swede and carrot into the saucepan and stir. Leave to simmer for 5 minutes.
Add in the red lentils and 250ml of water, leaving to simmer for 30 minutes.
Whilst the mixture is simmering, in a separate pan boil the potatoes until soft. Strain the potatoes, add the vegetable margarine and mash.
In an oven-proof dish, add the lentil and vegetable mixture, adding the mashed potato on the top and cook in the oven for 25-30 minutes.