1. Bring a large pot of water to a boil. Add the shrimp and cook for 2-3 minutes, until pink and opaque. Drain and transfer to a mixing bowl.
2. In a separate bowl, combine the vinegar, water, salt, sugar, garlic, mustard seeds, red pepper flakes, and black peppercorns. Stir until the salt and sugar are dissolved.
3. Add the red onion, lemon slices, and dill to the shrimp. Pour the vinegar mixture over the shrimp and stir to combine.
4. Transfer the shrimp and pickling liquid to a jar. Seal and refrigerate for at least 24 hours before serving.