Preheat the oven to gas 4,180°C, fan 160°C. Scrub the sweet potatoes, deseed the peppers and cut it all into 3cm chunks. Peel and quarter the onions, then break into petals. Peel the garlic clove, then finely slice half.
Place the veg in a large dry casserole pan on a high heat and cook for 5 mins so it starts to char.
Pick the coriander leave and put aside, the finely slice the stalks. Stir into the pan with 1 tbsp oil and the sliced garlic, and cook for 5 mins more, or until the veg is nicely charred, stirring regularly.
Stir through the tikka paste for 2 mins, then tip in the tomatoes, plus 1 tin’s worth of water. Add the butter beans (juice and all), then transfer to the oven for 45 mins or until the liquid has reduced and the sauce has thickened.
Meanwhile, finely grate the remaining garlic into the yoghurt. Squeeze in the lemon juice, mix well and season to perfection.
Ripple the yoghurt through the sauce, then roughly chop the coriander leaves and scatter them over the curry. Delicious served with rice and poppadoms.