Using a tenderizer, flatten the meat to 5mm thickness. If the breast is too big, cut it in half vertically.
Put the garlic, olives, capers, anchovy and herbs into a small food processor and blend until you get a thick, coarse paste. Add a drop of olive oil if it’s too dry.
Season the chicken with salt and pepper and spread the paste on top. Roll them up carefully, then secure them with toothpicks or small meat skewers.
Heat some olive oil in an oven-proof pan, or casserole dish. Brown the chicken rolls, turning them around regularly and carefully. Transfer them to a plate.
Add more oil to the pan and gently and fry the onion until soft. Add the mushroom, stir until soft then pour in the stock. Add the rosemary and bring to the boil. Reduce it by half or until you get a thick gravy-like sauce.
30 minutes before serving, turn on the oven, set to 180°C. Put the chicken rolls in the mushroom sauce, spoon some sauce on top of each, then bake for about 15-20 minutes, until the chicken is cooked through properly.
Serve warm with some chopped parsley scattered on top.