What you need to do, for 4 servings (obviously the more people the more breasts are needed) is to take a sharp knife, and cut a pocket from the upper part of the breast.
Try to make the picket as large as possible, so more filling goes in. Spice the outside with salt, pepper, and optionally even garlic powder.
Fine slice the mushroom, and toast them on 2 tbs of olive oil. Shred the cheese, and use 80% of it for filling the breasts.
Make sure to put inside as much cheese that you still have space for the mushroom. Then, when the mushroom is toasted, fill the chicken with them.
Use a toothpick to close the pockets, so the cheese won't flow out when it's melting. In the pan, you've used for the mushrooms, put some more olive oil, heat up, and toast the chicken breasts on each side until it gets white.
Then, place them on baking paper, top it with a few drops of Olive oil, so it doesn't dry out, and pepper after taste, and place it covered into a pre-heated oven at 200°C.
Leave it for about 25-30 minutes. Remove the cover, add on top the leftover cheese, and put it back in the oven for another 10-15 minutes.