Bring a large pot of water to a boil. Add shrimp and cook for 2-3 minutes until pink and opaque. Drain and set aside.
In a mixing bowl, combine vinegar, water, sugar, salt, garlic, mustard seeds, celery seeds, and red pepper flakes. Stir until sugar and salt are dissolved.
In a jar, layer the shrimp, red onion, and lemon slices. Pour the vinegar mixture over the shrimp until fully submerged. Seal the jar and refrigerate for at least 24 hours before serving.