Heat the olive oil in a large skillet over medium heat. Add the chopped onion, bell pepper, and celery. Cook, stirring occasionally, until the vegetables are softened, about 5-7 minutes.

Add the minced garlic and cook for another minute until fragrant. Then, sprinkle the flour over the vegetables. Stir constantly for 1-2 minutes, until the flour is lightly browned and forms a smooth paste (this is the roux). Continuously stir to prevent burning.

Gradually whisk in the chicken broth, adding a little at a time and whisking vigorously to prevent lumps. Once the broth is incorporated, stir in the heavy cream, Creole seasoning, Worcestershire sauce, and cayenne pepper if using.

Bring the mixture to a simmer, then reduce the heat to low. Simmer gently for 5-7 minutes, or until the sauce has thickened slightly to your desired consistency. Season with salt and pepper to taste.

If using butter, stir it into the sauce now. Gently place the red snapper fillets in the sauce. Don't overcrowd the pan - cook in batches if necessary. Cover the skillet and cook for 5-7 minutes, or until the fish is cooked through and flakes easily with a fork. Cooking time will depend on the thickness of the fillets.

Stir in the chopped parsley. Serve the red snapper immediately over cooked rice. Garnish with extra parsley if desired.