Combine all ingredients for the marinade in a bowl and mix well. Add shrimp. Stir and coat with the marinade on all sides. Let is rest 15 to 30 minutes in the refrigerator.
To a large pan, add the oil. Add the mustard seeds, curry leaves. Then add sliced onions, ginger, garlic and sauté for about 3 minutes until the onions are translucent.
Add tomato, spices and mix well. Cover with a lid and cook for 6-8 minutes until the tomatoes soften.
Add marinated shrimp. Stir well and let the curry cook for 5-7 minutes until shrimp is just cooked. Do not overcook.
Garnish with cilantro. Shrimp masala is ready. Enjoy with basmati rice and naan.