Heat oil in a large nonstick skillet over MEDIUM-HIGH heat. Add onion, ginger, and garlic to pan; cook for several minutes or until softened, stirring occasionally. Stir in diced tomatoes and tomato sauce. Add garam masala, paprika, curry powder, cumin, salt, and crushed red pepper. Bring to a boil, reduce heat to LOW and simmer while you prepare the chicken.
Cut chicken into bite-size chunks and place in a 6-quart slow cooker. Pour tomato mixture over chicken, cover and cook on LOW for 5 to 6 hours, or until chicken is very tender.
After initial cooking time, shake can of coconut milk well and measure out 1/4 cup. Add 2 tablespoons cornstarch to the measuring cup and stir until well combined and cornstarch has dissolved. Add the remaining coconut milk in the can to the slow cooker, stirring it into the chicken and sauce. Add the cornstarch slurry and stir until well combined. Cover and cook for an additional 15 minutes, or until heated through.
Garnish with cilantro and serve over rice. Top each serving with 1 tablespoon yogurt, if desired.