Peel the celeriac and cut it into 5mm thick long strips.
Cut the onion and fennel into thin slices.
Heat the oil, then gently sauté the onion, celeriac and fennel over low heat for 10 minutes under a lid. Stir occasionally.
Stir in the lemon juice and add the garlic slices (if used), then put the lid on again and cook for 5 more minutes for a few minutes.
Add the beans and a splash of water and season with salt and pepper. If using roasted garlic, add the soft flesh now and stir. Gently cook under cover for another 5 minutes, until the vegetables are tender.
Stir in the dill and serve.