Peel and thickly slice the onion. Peel and grate the ginger and garlic. Halve the chilli and flick out most of the seeds. Finely chop it. Peel the sweet potatoes and chop into bite sized cubes.
Roughly chop the tomatoes. Drain and rinse the chickpeas. Finely chop the coriander stalks and roughly chop the leaves. Keep them separate. Pop the cardamom pods into a mortar and pestle and lightly crush them.
Heat a heavy-based pan. Add 1 tbsp oil. Add the onion and cook for 3-4 mins till just softened. Add the ginger, garlic and chilli and cook with the onion for 1 min. Add 2 tsp tikka masala spice and the crushed cardamom pods and seeds and stir.
Tip in the sweet potato cubes and stir through the spiced onions. Add the chopped tomatoes, chickpeas, coriander stalks and 400ml boiling water. Clamp on a lid and cook for 15-20 mins till the sweet potato is just tender. Stir through half the pot of yogurt and cook for 5 mins more.
Finely chop half the cucumber. Peel and grate the remaining garlic. Stir the cucumber and garlic through the remaining yogurt. Add a pinch of salt and pepper. This is your raita.
Taste and season the chana masala. Spoon the chana masala into warm bowls. Top with a dollop of raita and the coriander leaves.