Stir together flour and oil in a cast-iron skillet over medium heat and cook until roux is the color of butter scotch about 10 minutes. Add onions, bell pepper, and celery and cook until softened, about 10 minutes; add garlic. Scrape roux and vegetables into a stock pot; add water and seafood stock base and bring to a boil, stirring until incorporated. Reduce heat to medium-low; add salt, black and cayenne pepper to taste and simmer uncovered for another 30 minutes. Stir in shrimp and green onions; cook until shrimp are opaque, about 3 minutes.

Adjust seasonings. Serve over rice.