Soak the nuts: Place cashews in a bowl, cover with boiling water, and soak while you prep the veg.
Prep the vegetables: Preheat oven to 200°C (400°F). Peel the sweet potato and dice into small chunks. They should be small as you'll be blending them up and it's easier to have them soft this way. Remove the head of the capsicum and slice lengthwise. Finely dice the onion and the deseed the tomato - you want to remove the excess water from the tomato for the masala.
Roast the vegetables: Toss sweet potato and capsicum in a little olive oil, salt and pepper. Spread on a lined baking tray and roast for 30 minutes, or until soft and slightly caramelised.
Make the sauce: Drain the soaked nuts. Add to a blender with roasted sweet potato, capsicum, smoked paprika, lemon juice, and ½ cup water. Blend until silky smooth. Add more water as needed for blending, but keep it thick.
Cook the onion-tomato masala: In a large pan, heat butter and oil over medium heat. Add cumin, cardamom, cinnamon, and cloves. Let sizzle for 30 seconds until fragrant. Add chopped onions and cook for 6–8 minutes until golden.Stir in ginger-garlic paste; sauté for 1 minute.Add tomatoes and salt. Cook for 5–7 minutes until softened and saucy.Add garam masala and ½ cup water. Simmer until slightly thickened.
Roast the chickpeas: Note: The chickpeas will fry up better when their skins are off. They come off easily if after your wash them, you rub them with a towel to remove. In a small pan, heat 1 teaspoon oil over medium heat. Add chickpeas and a pinch of salt. Fry for 5–6 minutes, stirring occasionally, until slightly crispy and golden. You can also use an air-fryer for this step instead if you want to save on the oil.
Combine and finish: Add crushed kasuri methi to the masala. Stir in the sweet potato–capsicum sauce. Fold through the crispy chickpeas. Add the drained and rinsed butter beans. Taste and adjust seasoning.
Serve: Spoon curry into bowls. Top with a dollop of coconut yogurt and a squeeze of lemon. Serve with steamed basmati rice and warm naan.