Preheat the oven to 220°C/200°C fan/Gas 7. Unwrap your duck and pat dry with kitchen paper. Dust with salt and pepper. Set a frying pan over a high heat. When hot, place the duck legs in, skin side down. Cook for 4 mins till the skin is golden. Flip them over and cook for 1 min.
Transfer the duck legs to a small baking dish. Pour 300ml boiling water around the legs. Cover with foil. Slide into the oven for 30 mins. Save the pan with the duck fat for later.
Peel and finely chop half the onion. Peel and grate the ginger and one garlic clove. Halve the chilli and flick out most of the seeds. Finely chop it. Crush the cardamom pods with the back of a knife to split the pods open. Flick out the seeds into a small bowl and set aside.
Set a saucepan on the heat for 1 min. When warm, drizzle in 1 tbsp olive oil. Add the chopped onion and cook for 5 mins till softened. Add the ginger, garlic, chilli and cardamom seeds. Add a good pinch of salt. Cook for 1 min. Tip in the lentils with 500ml boiling water. Stir and set the heat to low. Pop on a lid and cook for 30 mins. Add a splash of water if it's looking a little dry.
Finely shred the spinach. Stir the spinach through the lentils. Turn off the heat and place the lid back on. Leave to steam cook while you finish the duck. The daal should be thick and creamy, but not dry.
Finely chop the halved onion. Peel and grate the remaining garlic clove. Roughly chop the tomatoes. When the duck has 10 mins left to cook, set the frying pan you used to fry the duck legs on a medium heat for 1 min. Add the onion and cook for 3 mins. Add the garlic and cook for 1 min. Turn the heat to low.
Add the chopped tomatoes and cook those for 5 mins till softened and saucy. Remove the duck from the oven. Set the duck legs on a plate to cool slightly. Sprinkle the tikka masala spice mix into the tomatoes and stir. Pour the water from the baking dish into the pan. Pop on a lid. Cook on a gentle heat for 10 mins while you finish the dish.
Peel off the skin from the duck legs and discard. Using a sharp knife and a fork, cut away and shred the meat from the bone. Add it to the simmering pan. Heat for 4 mins. Stir in the cream. Taste and adjust the seasoning, adding more tikka masala spice mix if you like. Taste the lentils and season.
Serve the daal in shallow bowls with the duck tikka mankhani spooned on top.