Preheat the oven to 450F/250C
On the cutting board chop up rosemary leaves roughly. In a mortar combine chopped rosemary, garlic, a pinch of coarse salt and 1 tbsp of olive oil and bash it all together with a pestle until you have a paste, then add the rest of the olive oil and the juice of the lemons, reserve the lemon halves.
Arrange the chicken pieces and potatoes on a baking tray and spoon the rosemary mixture on top making sure it covers chicken and potatoes. Stick the lemon halves wherever you have free space on the tray, they will infuse more flavour into the dish. Cook for 25 minutes.
Take out of the oven and add mushrooms, baste chicken, potatoes and mushrooms with the juices form the bottom of the pan. Cook for additional 25 minutes.