Heat oil in a large skillet over medium heat.
Whisk in flour to create a roux, stirring constantly until it turns a dark brown color.
Add onions, bell pepper, and celery; sauté until softened.
Stir in garlic and cook for an additional minute.
Add the chicken broth and diced tomatoes, bringing the mixture to a simmer.
Mix in the Cajun seasoning, salt, and pepper.
Add the shrimp and cook until they turn pink, about 5 minutes.
Reduce heat and let the etouffee simmer for 10-15 minutes.
Serve over cooked white rice.
Garnish with fresh parsley, if desired.