Peel the shrimp and place the shells in a medium saucepan. Cut the shrimp into 1-inch pieces and refrigerate until ready to use. Cover the shells with 4 ½ cups|1065 ml water and add 1 teaspoon of Tony’s Cachere, ½ teaspoon of Zatarain’s seasoning, and 2 bay leaves. Bring to a low simmer and cook for 25 to 28 minutes. Strain, discarding the shells, and set aside.
Meanwhile, melt the butter in a large saucepan over medium-high. Add the celery, bell pepper, and onion and cook 2 to 3 minutes, then add the garlic and cook 3 minutes more, or until the vegetables are soft. Transfer to a bowl and set aside. Wipe the pan clean.
Add the duck fat to the saucepan and reduce the heat to medium. Add the flour and cook, stirring with a whisk, until the flour has turned a shade darker than peanut butter, about 8 to 10 minutes. Add the vegetables back to the saucepan, then stir in 2 cups|473 ml of the reserved shellfish stock and the remaining bay leaf. Bring to a simmer and cook until slightly reduced, about 6 to 7 minutes. Add in the reserved shrimp, the crawfish, cream, and a few dashes of hot sauce and Worcestershire sauce. Cook until the shrimp are cooked through, about 5 minutes more. Season with the remaining 1 teaspoon Tony Cachere’s and 1 teaspoon Zatarain’s, the cayenne, salt, and pepper. Squeeze in the juice from the lemon and stir. Keep warm.
Meanwhile, bring a large pot of generously salted water to a boil. Add the pasta and cook until al dente, about 12 minutes. Drain, then add to the etouffee along with the parsley and thyme. Toss to combine and divide among plates. Garnish with the chives and serve.