Heat your oven to 200°C/Fan 180°C/Gas 6. Peel and finely chop the onion. Peel the carrot and potato and dice them both.
Pour 1 tbsp olive oil into a pan and warm over a medium heat. Slide in the onion, potato, carrot, 1 tsp Demerara sugar, 2 tbsp daal spices and a good pinch of salt and pepper. Fry for 5-6 mins, stirring now and then.
Drain the lentils and add them to the veg. Pour in 200ml water. Cover and simmer gently for 10 mins or till everything is tender.
While the lentils simmer, place the haddock fillets on a baking tray and season well with salt and pepper. Rub 1 tbsp olive oil over the fillets. Slide into the oven and roast for 10-12 mins till the fish is just cooked through.
Shred the mint leaves. Spoon 2 tbsp crème fraîche into the lentil pan and stir through, along with half the mint leaves and the juice from half the lemon (cut the other half into wedges).
Spoon the lentils into deep bowls and top with the haddock fillets. Serve with the remaining crème fraîche on the side and lemon wedges for squeezing, garnished with the last of the mint.