Combine oil and flour in a Dutch oven; cook over medium heat, stirring constantly, until roux is the color of a copper penny. Add onion and garlic; cook 5 minutes, stirring frequently.
Add 6 cups water, ham hock, parsley, salt, pepper, and beans; bring to a boil. Reduce heat; cover and simmer 1 1/2 hours or until beans are tender and a thick gravy has formed. If necessary, add more water to prevent beans from sticking.
Mound rice in the middle of serving plates. Spoon beans around rice, and sprinkle with chopped green onion.