In a large pot of boiling salted water, simmer the shrimp until just white throughout, about 2 minutes. Drain and transfer to an 8-by-11-inch glass or ceramic dish. Scatter the onion and jalapeño over the shrimp.
In a small saucepan, combine the vinegar, water, sugar, garlic, bay leaf, peppercorns and mustard seeds and bring to a simmer; stir until the sugar is dissolved. Pour the pickling liquid over the shrimp and let cool. Cover and refrigerate for at least 2 hours or up to 6 hours.
Using a slotted spoon, transfer the shrimp with the onion and jalapeño to a medium bowl. Add the olive oil and season with salt, then toss and serve.