In a large skillet, melt the butter over medium-high heat.
Add the onion, celery and bell pepper; cook until softened; about 7 minutes.
Add the garlic and cook until fragrant; 1 more minute.
Sprinkle the flour over the ingredients and stir constantly until lightly toasted; about 5 minutes.
Add the tomato paste, and stir; cook for 1 more minute.
Pour in the stock and 3/4 cup water.
Cook until the liquid is reduced by half; about 3 minutes.
Add the hot sauce, bay leaf, Creole seasoning, 1 teaspoon salt and 1/2 teaspoon pepper.
Bring to a boil, and cook until the mixture starts to thicken; about 2 minutes.
Stir in the shrimp and reduce the heat to medium.
Simmer until the shrimp is cooked through and opaque; about 5 minutes; turning each piece halfway through.
Remove from the heat; taste and adjust seasoning.
Sprinkle the scallions on top and serve, warm, over rice.