Heat oil in (about 3 TBSP) in large skillet. Add and garlic + onions and cook until brown and translucent. Add cayenne, turmeric, cumin powder, salt and cook until the masala is well incorporated. Add 3-4 chopped tomatoes and cook until completely soft and brown. Add cilantro and green chilies and cook 3-4 mins. At this point, you can add more salt or cayenne pepper to taste if you want it spicier. Add shrimp and cook about 6 minutes until it is tender.