Melt the butter in a large skillet over medium heat. Stir in the flour and cook, stirring often, until the mixture turns a light brown color, about 5 minutes.
Add the onion, green bell pepper, and celery to the skillet. Cook for 4 minutes until the vegetables soften. Stir in the garlic and Cajun seasoning and cook for 1 minute more.
Pour in the shrimp stock and bring the mixture to a simmer. Let it cook for 10 minutes, stirring now and then, until the sauce thickens.
Add the shrimp to the skillet. Cook for 3 to 4 minutes until the shrimp turn pink and opaque.
Spoon the étouffée over the cooked white rice. Place sliced black olives on some of the shrimp to create eyes and scatter the chopped green onions on top.