Heat the olive oil in a large pot over medium heat. Add the onion and cook until softened, about 5 minutes. Add the garlic and cook for 1 minute more.
Add the Brussels sprouts and shiitake mushrooms and cook until slightly softened, about 5-7 minutes.
Pour in the vegetable broth, thyme, salt, and pepper. Bring to a boil, then reduce heat and simmer for 30 minutes, or until the Brussels sprouts are tender.
If desired, stir in the cooked white beans during the last 5 minutes of cooking.