Soak beans overnight in a bowl. Drain and rinse them before cooking.
Warm a heavy-bottomed 12-quart pot over medium heat for 2 minutes, then add bacon grease and heat for 30 seconds. Add sausage and cook, turning as needed until browned on all sides, about 12 minutes.
Trim and discard the root end and very top of the green onion. Chop the remaining part of the onion, separating into two piles.
Add green onion tops, yellow onion, celery, garlic and bay leaf; stir to combine. Add beans and pork meat, and cover with water. Cover pot and bring to a boil over high heat.
Boil for 10 minutes then reduce heat to a low simmer and cook, uncovered, stirring often, until the beans are tender, scrapping the bottom of the pot, about 2 hours.
Turn off heat. Remove and discard bay leaves.
Taste and season with salt and pepper. Garnish with green onion bottoms. Serve beans over rice or jambalaya.