Melt the butter in a large skillet over medium heat. Add the onions, bell peppers, and celery and saute until soft and golden, about 10 minutes. Add the garlic and cook for 2 minutes. Add the shrimp, salt, and cayenne and cook for about 4 minutes, or until the shrimp turn pink.

Dissolve the flour in the water and add to the shrimp mixture. Stir until the mixture thickens slightly. Reduce heat to medium-low and simmer for 6 to 8 minutes, stirring occasionally. Add the parsley and green onions. Stir and cook for about 2 minutes more.

Serve immediately.