Microwave onion, oil, serrano, tomato paste, garam masala, garlic, ginger, sugar, and ½ teaspoon salt in bowl, stirring occasionally, until onion is softened, about 5 minutes; transfer to slow cooker. Stir in tomatoes. Season chicken with salt and pepper and stir into slow cooker. Cover and cook until chicken is tender, 2 to 3 hours on low.
Whisk ½ cup sauce and yogurt together in bowl (to temper), then stir mixture back into slow cooker and let sit until heated through, about 5 minutes. Stir in cilantro and season with salt and pepper to taste. Serve.
