Rinse the fennel bulbs, peel off the fine outer skin and cut the bulbs into thin slices. Peel and finely dice the onion.
Rinse the long grain rice and drain well. Bring the clear soup to the boil in a saucepan. Heat the oil in a second saucepan. Add the diced onion and fennel ribs and sauté over low heat, stirring throughout, until the fennel pieces begin to soften. Add the long-grain rice and sauté until lightly browned. Add a small ladle of the boiling clear soup and stir until the long-grain rice has absorbed the liquid. gradually add a tiny bit of clear soup to the long-grain rice form at a time. Cook the long grain rice at low temperature for about 20 minutes. Mix the parmesan into the finished long-grain rice and let it sit for 3 minutes. Serve sprinkled with fennel greens.