Step 1Rinse the shrimp and pat them dry on paper towels. Put them in a bowl and sprinkle with the cayenne, turmeric, mustard and lemon juice. Stir gently to coat the shrimp evenly with the spices. Cover and refrigerate for 30 minutes.
Step 2When the shrimp have marinated, combine the oil, cumin seeds and curry leaves in a large wok or skillet over medium high heat. Cover, if using mustard seeds, because the seeds will splatter and pop. Cook until the cumin darkens or you hear the mustard seeds crackle, 1 to 2 minutes.
Step 3Add the shrimp and cook 30 seconds, stirring often. Add the green onion and cook, stirring, until the shrimp turn pink all over, about 1 to 3 minutes. Sprinkle with salt to taste and serve hot.