Heat the vegetable oil in a large skillet over medium heat.
Add the ground sausage to the skillet and cook until browned, breaking it apart with a spatula, about 5-7 minutes.
Stir in the diced onion, bell pepper, celery, and garlic. Cook until the vegetables soften, about 5 minutes.
Add the rice, Cajun seasoning, black pepper, salt, and cayenne pepper (if using). Stir to combine and let the rice toast for about 2 minutes.
Pour in the chicken broth and bring the mixture to a boil.
Once boiling, reduce the heat to low, cover the skillet with a lid, and let it simmer for 20 minutes, or until the rice is cooked and has absorbed the liquid.
Remove the skillet from the heat and let it sit covered for an additional 5 minutes.
Fluff the rice with a fork and garnish with sliced green onions before serving.