Heat the oil in a skillet over medium heat. Finely chop the onions and garlic in a blender or food processor, and cook in the skillet until tender. Puree the tomatoes and green bell pepper in the blender or food processor, and mix into the skillet. Season with salt, garam masala, and chili powder. Stir in the cream and butter, and continue cooking until heated through.
Mix the chicken into the skillet. Cover, and simmer 20 minutes, or until chicken juices run clear.