Put 2 branches of Rosemary over 1 Kg of Fillet and tie it with kitchen twine. Brown it in a pan adding a bit of oil, sprinkle with salt, pepper and chopped Rosemary.
Transfer the fillet in a baking dish with 5-6 tablespoons of extra virgin olive oil,4 tablespoons of broth and 2 -3 of balsamic vinegar and cook in the oven at 220°C for about 10 minutes. Turn down to 200°C and continue cooking for another 12-15 minutes.
Switch off and leave the fillet in oven for another 5-10 minutes. Put the pan on the stove, remove with a spoon the caramelized parts from the bottom, add 3-4 tablespoons of broth, 2-3 of balsamic vinegar of Modena IGP, 1 spoon of flour and then shrink it.
Cut the roast beef into slices of about 1 cm thick and serve on a plate sprinkling the slices with the obtained sauce.