Zest and juice 1 lemon and reserve in a small bowl; reserve the lemon rinds separately. Cut the second lemon into quarters.
In a large stockpot, heat the oil over medium heat. Add the sausage, onion, celery, and garlic halves, and saute until the onion begins to turn golden and the garlic is fragrant, 5 to 7 minutes. Add 1 gallon of water and the lemon rinds and quarters. Add the Creole seasoning, salt, thyme, parsley sprigs, and bay leaves; bring the mixture to a boil.
Add the potatoes and corn, cover, and cook until the potatoes are tender when pricked with a fork, 10 to 12 minutes.
Meanwhile, in a small saucepan, melt 2 tablespoons of butter over medium heat. Add the finely chopped garlic, and cook until fragrant, 30 seconds. Do not let brown. Add the remaining 10 tablespoons butter and melt. Remove from the heat and skim off any white foam that floats to the top. Stir in the pepper, chopped parsley, and reserved lemon zest and juice. Keep warm.
When the potatoes are tender, stir the shrimp into the stockpot, and let cook for 1 minute. Remove the stockpot from the heat. Use a large slotted spoon to scoop the shrimp and vegetables out of the pot and onto a serving platter or large baking sheet. Spoon about 1 cup of the boil liquid over the shrimp and vegetables.
Serve with the butter sauce, hot sauce, and an extra sprinkling of Creole seasoning.