In a large pan on medium heat, add oil, onion, salt and pepper. Sauté for about 3 minutes. Add garlic and ginger, cook until fragrant. Next add cumin seeds, mustard seeds, turmeric, red chili powder, and coriander powder. Stir into the onions then add a few chilis (optional). Add the chicken to the pan (that has been cut into bite size pieces). Season with salt and pepper. Cook chicken for about 5 minutes then add tomatoes and a splash of water. Cover and simmer the curry on low heat for about 7-10 Minutes. Once you have a nice sauce and the chicken is cooked to at least 165 degrees internally, add gram masala powder, fresh chopped coriander, and adjust the salt and seasoning to your taste. Serve it up with naan bread or basmati rice. Enjoy!
