Heat oil and rosemary in a non-stick pan and let the meat brown on all sides.
In the meantime, wash 1 orange well and cut it into thin slices.
Place the meat on a sheet of baking paper, season with salt and pepper and cover with the orange slices, then close the baking paper and tie it with string to keep it closed.
Cook for 40 minutes in a fan oven preheated to 180°C.
Open the foil and cook for another 15 minutes at 200°C, then remove from the oven and leave to rest for 10 minutes before cutting.
Transfer the cooking juices into a saucepan and add the broth and the juice of half an orange and leave to simmer for a couple of minutes.
Mix the butter with the flour and add it to the saucepan, then cook until it thickens slightly.
Wash, clean and slice the fennel.