Bring a large pot of generously salted water to a boil. Add the shrimp and cook for 2 minutes, until just pink. Drain and immediately plunge into an ice bath. Drain well and pat dry.

In a small saucepan, combine the vinegar, water, sugar, salt, and pickling spice. Bring to a simmer, stirring until the sugar and salt dissolve. Remove from heat and let cool slightly.

In a large glass jar or deep bowl, layer the shrimp, red onion, lemon slices, and garlic cloves.

Pour the warm brine over the shrimp mixture to cover completely. Let cool to room temperature, then cover and refrigerate for at least 2 hours, preferably overnight.

Use a slotted spoon to transfer the shrimp, onion, and lemon to a serving bowl. Drizzle with a little brine and serve cold.