Heat the olive oil in a saucepan over medium-low heat. Add the diced onion and cook for 5 minutes, until softened.
Stir in the garlic, ginger, cumin, coriander, turmeric and garam masala. Cook for 30 seconds until fragrant.
Add the chopped tomatoes, chickpeas, finely diced celery and water. Stir to combine, bring to a gentle simmer and cook for 15 minutes.
Meanwhile, cook the diced chicken thigh in a separate pan until cooked through, with no pink remaining in the centre. Set aside.
Carefully blend the tomato, chickpea and celery mixture until smooth.
Return the sauce to the saucepan and stir in the coconut milk, cream, chopped cashews, brown sugar and vinegar.
Finish the curry. Add the cooked chicken and simmer gently for 3–5 minutes, stirring occasionally, until heated through and the flavours have combined. Stir in the butter at the end.
Cook the ancient grains rice according to the packet instructions.
Serve the creamy butter chicken curry over the rice and enjoy!