Heat oven to 180C/160C fan/gas 4 and heat half the oil in a large frying pan. Brown the beef in batches, and transfer to a flameproof casserole dish. Add the remaining oil to the pan with the onions, and fry gently until softened. Stir in the lemongrass, ginger, tamarind, garlic, turmeric, curry powder and sugar, and fry for a few mins until fragrant. Add the coconut milk with 1 can of water, stir into the beef and bring to a simmer. Cover and bake for 1 hr. Meanwhile, toast the cashews in a dry frying pan until golden.
After 1 hr, stir in the new potatoes and cook for 1 hr more until the potatoes are tender, removing the lid halfway. The casserole can now be cooled and chilled, or frozen, if preparing ahead.
Put the casserole back onto the hob. Stir in the bananas and garam masala and simmer, uncovered, for 10 mins until the sauce has thickened.
Cook the rice following pack instructions, then mix with the cashews, crispy onions and some salt. Scatter the curry with coriander, season well, then taste – it may need a squeeze of lemon to finish it off.