Once the oil is just heated, add chopped ginger and garlic and cook on low flame for 2-3 minutes. Don't let the ginger & garlic burn.
When ginger and garlic are fragrant, add chicken, along with black pepper, and turn the heat to medium -high.
Cook for a few minutes on medium-high heat until chicken is white on the outside, no longer pink.
Add 1 1/2-2 cups of crushed tomatoes or fresh tomato sauce. (see note)
Turn the stove to low heat, until chicken is fully cooked on the inside.
Turn off the stove, sprinkle garam masala and mix.
Garnish with ginger, cilantro, and chopped green chilis.
Notes
Note: If using fresh tomatoes, chop 8-10 plum tomatoes. In a separate pot, add the chopped tomatoes, and allow them to cook for 10-15 minutes until saucy.