1. Trim the fronds from the fennel bulbs and reserve.
2. Cut the bulbs in half vertically, then slice each half horizontally.
3. Heat the olive oil in a large heavy pan.
4. Add the fennel slices and onion and saute over a low heat for 6 to 8 minutes. Stir frequently.
5. Add the water, salt and pepper and raise the heat to medium. Cook for 25 to 30 minutes.
6. Serve with the cooking liquid in a deep dish, garnished with fennel fronds.