Thoroughly combine the seasoning mix ingredients in a small bowl and set aside. In a separate bowl combine the onions, celery and bell peppers.
In a large heavy skillet (preferably cast iron), heat the oil over high heat until it begins to smoke, about 4 minutes. With a long-handled metal whisk, gradually mix in the flour, stirring until smooth. Continue cooking, whisking constantly, until roux is dark red-brown, about 3 to 5 minutes (be careful not to let it scorch in the pan or splash on your skin). Remove from heat and immediately stir in the vegetables and 1 tablespoon of the seasoning mix with a wooden spoon; continue stirring until cooled, about 5 minutes.
In a 2-quart saucepan bring 2 cups of the stock to a boil over high heat. Gradually add the roux and whisk until thoroughly dissolved. Reduce heat to low and cook until flour taste is gone, about 2 minutes, whisking almost constantly (if any of the mixture scorches, don’t continue to scrape that part of the pan bottom). Remove from heat and set aside.
Heat the serving plates in a 250°F oven.
In a 4-quart saucepan melt 1 stick of the butter over medium heat. Stir in the crawfish (or shrimp) and the green onions; sauté about 1 minute, stirring almost constantly. Add the remaining stick of butter, the stock mixture and the remaining 1 cup stock; cook until butter melts and is mixed into the sauce, about 4 to 6 minutes, constantly shaking the pan in a back-and-forth motion (versus stirring). Add the remaining seasoning mix; stir well and remove from heat (if sauce starts separating, add about 2 tablespoons more of stock or water and shake pan until it combines). Serve immediately.