Combine lemon juice, salt, Cajun seasoning, sliced onion, cider vinegar, oil, ice water, chili flakes, and black pepper in a large bowl that will later hold the shrimp.
Stir the mixture until everything is well combined and refrigerate to chill while you prepare the shrimp.
In a large pot, bring 8 cups of water to a rolling boil.
Halve the lemon, squeeze its juice into the boiling water, and drop the lemon halves into the pot.
Add the salt, Cajun seasoning, bay leaf, and black pepper to the pot, stirring to distribute the flavors evenly.
Carefully add the shrimp to the boiling water and cook them for 3 minutes, ensuring they are just cooked through.
Using a slotted spoon, transfer the cooked shrimp directly into the chilled pickling liquid, stirring them gently to coat.
Refrigerate the shrimp in the pickling liquid for at least 20 minutes to let the flavors meld.
Just before serving, stir in the fresh herbs and give the shrimp one final mix to evenly distribute the herbs.
The pickled shrimp with herbs is ready to be served.