Heat oil and butter in a pot over medium heat. Add the onion and fry until softened, about 8 minutes. Add ginger and fry for 2 minutes. Add the garam masala, garlic powder, cumin, turmeric powder and black pepper, and fry for 2 minutes. Then add crushed tomatoes, and cook for 2 minutes.
Add the chicken and salt, mix well. Add 2/3 cup warm water, cover the pot and simmer over medium-low heat until chicken is cooked through, about 45 minutes. Add more water during cooking, if necessary.
Once chicken is fully cooked, combine the sour cream and 1/2 cup water until well combined and pour in stew, mix well. Cover the pot and cook for 10 more minutes. Add the lemon juice. Taste and adjust seasonings. Serve Chicken Masala with bread or rice.