Soak the lima beans overnight in a small pot with 4-5 cups water. Drain. In a large soup pot, heat the oil and butter over medium heat. Saute the celery, carrots and onion for 5 minutes, until softened. Add the garlic and saute for 30 seconds. Add in the broth, mustard and thyme leaves and stir to combine. Stir the lima beans in. Bring to a boil, then reduce heat to a simmer and cook until the lima beans are softened, about 25-40 minutes.Add salt and pepper to taste. Serve with bread for dipping if desired.