The secret ingredient is... allspice!
In a large heavy pot, heat the oil over high. Whisk in the flour. (It will immediately begin to sizzle.) Reduce the heat to medium and continue whisking until the sauce turns deep brown, about 15 minutes. Using a wooden spoon, stir in the onions. Reduce the heat to medium-low and cook, stirring constantly, until the sauce turns dark brown and glossy, about 10 minutes more.
Add the celery, garlic, allspice and cayenne. Cook, stirring often, until fragrant, about 5 minutes. Add the tomatoes and stock and bring to a boil over high heat. Reduce the heat to medium and simmer, stirring often to prevent the sauce from burning or sticking to the pan, until the flavors meld, 5 to 7 minutes.
Reduce the heat to low and stir in the butter. Add the shrimp and scallion. Season with the Tabasco, salt and pepper and cook, stirring often, until the shrimp are cooked through, 2 to 3 minutes. Serve over the rice.
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