Place the lamb and the onion in a broiler pan ( I use this one a lot!) under the heated broiler on high for ⅚ minutes, or until the meat is brown outside but still pink inside.
Sprinkle with sea salt and freshly ground pepper.
In the meantime melt the butter in a skillet, add the sliced fennel, and saute' for few minutes.
Add a pinch of sea salt.
Add the water to the skillet and cover with a tight lid.
Cook for about 10 minutes, until tender but not overcooked.