Heat oil in an enamel-coated cast-iron Dutch oven over medium heat. Add onion; cook, stirring occasionally, until brown and starting to caramelize, about 10 minutes.
Reserve rice packet from Holiday Bean Soup Mix; stir beans and seasonings from soup mix into pot with onions. Add 11 cups water and ham hock, and bring to a boil. Reduce heat to medium-low, partially cover, and cook until beans are mostly tender, about 3 hours. (Add remaining 1 cup water to keep beans covered by ½ inch of water, if needed.)
Stir rice packet and salt into pot. Cook until rice is tender, about 20 minutes, adding additional water if needed. Remove ham hock; shred meat, discarding bone. Return meat to soup. Garnish servings with green onion and lemon, if desired.
Recipe by Southern Cast Iron at https://southerncastiron.com/holiday-bean-soup/