Place the onion, chiles and cashew nuts in a small saucepan. Bring to a boil and reduce to a simmer for 10 minutes.
Remove the onion, chiles and cashew nuts. Place the onions, HALF the chiles and cashew nuts in a blender or small food processor. Puree for 2 - 3 minutes until a smooth paste is formed, adding a bit of the cooking water if needed to make a smooth paste.
Taste the puree. If it's spicy enough, discard the remaining chiles. If not, add more chile and puree until the paste is spiced to your liking.
In a large saute pan, heat the pan over medium-high heat. Blot the chicken dry with paper towels. Add the oil and swirl to coat the pan. Add ginger and galic paste (or grated ginger and garlic) and cook for 1 minute. Add the chicken and cook until the chicken is no longer pink on the outside, about 3 - 4 minutes.
Add the coriander, cumin, garam masala, turmeric and salt. Cook for another minute. Add the onion puree and stir to combine. Add two tablespoons of water to the blender or food processor, swirl and pour into the pan. Cook for another 10 - 15 minutes until the chicken is cooked through and the sauce has thickened.
Taste the sauce and adjust the seasonings. Garnish with a drizzle of cream and a sprinkling of garam masala and mint leaves if desired. Serve.