Dissolve 3 tablespoons salt in 4 quarts cold water in a large container. Add beans and soak at room temperature for at least 8 hours or up to 24 hours. Drain and rinse well.
Heat oil in Dutch oven over medium heat until shimmering. Add onion, ½ teaspoon salt, and ⅛ teaspoon pepper. Cook until softened and lightly browned, 5 to 7 minutes. Stir in oregano and cook until fragrant, about 30 seconds.
Stir in broth, celery, and soaked beans and bring to boil. Reduce heat to low, cover, and simmer until beans are tender, 45 to 60 minutes.
Stir in lemon juice and Aleppo pepper. Season with salt and pepper to taste.
Serve, drizzling individual portions with extra oil and sprinkling with fresh parsley.
