Cover beans with soaking water; refrigerate 8 to 12 hours. Drain; rinse. Prep time includes 8-hour soak. Heat oven to 400°F.
Cut top from garlic; drizzle oil, foil-wrap, and roast 40 to 45 minutes until soft. Meanwhile, full rolling-boil beans in fresh boiling water 30 minutes; drain.
Put beans, ham bone, celery root, and onion in 7-quart slow cooker. Squeeze roasted garlic from skins into cooker; add soup water, sage, and pepper.
Cover; cook on low 6 to 7 hours until beans very tender and meat loosens. Lift bone and celery root out.
Immersion-blend celery root with 1 cup broth in tall heatproof container; return puree. Pull usable meat from bone; discard bone and gristle and return meat.
Add spinach; cover and cook on high 15 to 20 minutes until greens softened and ham 165°F. Add lemon juice; taste before adding up to measured salt. Divide among eight bowls.