While the bouillon dissolves, begin heating the oil and butter in a 12-inch nonstick skillet over medium heat. Peel and coarsely chop the onion, adding it to the skillet as you chop. Stir frequently. Rinse and dry the celery and dice it. Add the celery to the skillet. Stir frequently. Dice the green pepper half (removing any ribs and seeds) and add it to the skillet. Cook the vegetables, stirring constantly, for 1 minute.
Stir in the garlic, Cajun seasoning, Worcestershire sauce, basil and thyme. Add the shrimp. (If the shrimp are raw, cook them, stirring frequently, until they are opaque throughout. This will take about 3 minutes or longer, depending on the size of the shrimp.)
Sprinkle the flour evenly over the skillet; cook, stirring constantly, until the flour is absorbed and forms a paste, about 1 to 2 minutes. (Some of the flour will stick to the bottom of the skillet.) Stir the reserved bouillon again to recombine it with the water and add it to the skillet. Cook and stir until no lumps of flour remain, scraping all of the flour off the bottom of the pan. Continue to cook and stir until the sauce thickens to the consistency of heavy cream, about 2 minutes. Taste the sauce and add cayenne pepper or Tabasco sauce if desired. Remove the skillet from the heat and serve at once, over hot rice, if desired.