Peel the shrimp or crawfish and use the shells to make the stock. If you can't buy whole raw shrimp where you live, go ahead and use peeled ones and substitute vegetable stock for the seafood stock.
Combine the onions, celery and bell peppers in a bowl and set aside.
Heat the oil in a large, heavy skillet (preferably cast iron) over high heat until it begins to smoke, about 4 minutes. Gradually whisk in the flour, stirring until smooth. Continue cooking, whisking constantly, until the roux is dark red-brown, about 3 to 5 minutes, being careful not to let it scorch or splash on your skin. Remove from the heat and immediately stir in the vegetables and 1 tablespoon of the Seafood Magic (or other Magic Seasoning) with a wooden spoon. Continue stirring until cool, about 5 minutes.
Bring 2 cups of the stock to a boil in a 2-quart saucepan over high heat. Add the roux by spoonfuls to the boiling stock, stirring until dissolved between each addition. Reduce the heat to low and cook, whisking almost constantly, until the flour taste is gone, about 2 minutes. If any of the mixture scorches, don't continue to scrape that part of the pan bottom. Remove from the heat and set aside.
Melt 1 stick of the butter in a 4-quart saucepan over medium heat. Stir in the shrimp (or crawfish) and the green onions, and sauté, stirring almost constantly, for 1 minute. Add the remaining butter, the stock mixture and the remaining 1 cup stock. Cook, constantly shaking (versus stirring) the pan in a back-and-forth motion, until the butter melts and is mixed into the sauce, about 4 to 6 minutes. Add the remaining Seafood Magic (or other Magic Seasoning), stir well, and remove from the heat. If the sauce starts to separate, add 2 tablespoons more stock or water and shake the pan until it combines. Serve immediately over the rice.