Pass around a small pitcher of sherry and invite your guests to season their servings to taste.
Soak beans overnight in cold water to cover. (Drain or not, as you prefer.) Add beans to soup kettle with water, bring to a boil, reduce heat, and simmer, uncovered, 2 hours or until beans are tender. Add more water as necessary. Heat oil in skillet and saute onions, celery, garlic, cumin, and oregano until tender. Add to beans and simmer 30 minutes. Press through food mill. Return to kettle, Season with salt and pepper, and reheat slowly. Garnish with lemon slices.