Boil onions and cashews in water for 10 minutes, cool down, and blend into a fine paste.
Marinate chicken with yogurt, half the black pepper, and salt for 20 minutes.
Melt butter in a pan, sauté ginger paste, garlic paste, and onion-cashew paste for 5 minutes.
Add marinated chicken and cook on high heat until sealed.
Add water, remaining black pepper, and garam masala; cover and simmer for 20 minutes until chicken is tender.