Place a large Dutch oven, preferably cast iron, over medium-high heat. Add the sausage and cook, turning once, until browned on both sides. Transfer to a platter and set aside. Reduce heat and add cooking oil to make 1/2 cup; whisk in flour and cook, stirring constantly, until a dark brown roux is achieved (do not burn). Add onion, bell pepper, and garlic and cook, stirring occasionally, until onion is translucent, about 10 minutes.

Add shrimp, bring to a simmer, lower heat, cover, and simmer for 5 minutes. Add 1 cup of water, salt, black pepper, and cayenne pepper to taste and simmer for 30 minutes, adding browned sausage to the pan for the last 10 minutes. If stew is too thick, stir in more water. 

Taste and adjust seasonings, garnish with green onions and parsley, and serve over hot cooked rice.