Place beans in a large soup pot, cover with cold water, and let sit overnight. Drain and rinse beans.
A quick-soak method: cover dried beans with cold water; bring just to a boil. Remove from heat; cover and let stand 1 to 2 hours. Drain and rinse beans.
In the same soup pot over medium-high heat, bring the 3 quarts of water to a boil. Reduce heat to low; add beans and ham bone or ham. Cover pot and let simmer 1 hour.
In a large frying pan over medium-high heat, melt butter. Add onions, celery, and garlic; saute until vegetables are soft. Add vegetables and mashed potatoes to the cooked soup and let simmer an additional 1 hour or until beans are thoroughly cooked.
Remove ham bone or ham piece. Cut ham into small pieces, removing bone and fat; return to soup pot. Add salt and white pepper.
Serve in soup bowls and garnish with chopped parsley.
Makes 6 servings.
Senate Bean Soup Recipe: https://whatscookingamerica.net/Soup/SenateBeanSoup.htm