In a small skillet, toast the peppercorns over moderate heat until fragrant, about 30 seconds; let cool.
Transfer the peppercorns to a mortar or spice grinder and grind to a powder.
Put the shrimp in a bowl, toss with 1 teaspoon of the ground peppercorns, and season with salt.
In a medium skillet, heat 1 tablespoon of the vegetable oil.
Add the shrimp and stir-fry over moderate heat until almost cooked through, 4 minutes.
Transfer to a plate.
Heat the remaining 3 tablespoons of vegetable oil in the skillet.
Add the chopped scallions, garlic, jalapeños, and chile and cook over moderate heat, stirring, until the scallions and garlic are softened, 5 minutes.
Add the remaining ½ teaspoon of ground peppercorns and cook, stirring, for 1 minute.
Add the shrimp and lime juice and stir until the shrimp are just cooked through, 1 minute.
Season with salt and transfer to a bowl.
Garnish with the sliced scallion, drizzle with the chile oil, and serve.
