Boil enough water to generously cover all of the shrimp. Peel and devein the shrimp, leaving the tails on, and boil the shrimp for no more than three minutes or until the shrimp turn pink, the drain the water.
While the shrimp are boiling, mix the oil, cider vinegar, ketchup, tarragon vinegar, lemon juice, Worchestershire sauce, bay leaves, sugar, salt, and dry mustard in a bowl. When your shrimp are ready, take the sliced onions and layer them with the shrimp in a large, shallow dish before pouring the oil mixture over them.
Leave in the refrigerator to marinate for up to 2 days, stirring occasionally, Discard the bay leaves before serving.