Cook mushrooms: In a large non-stick pan heat 1 tablespoon of oil, add the mushrooms and cook over a fairly high heat until lightly browned and all the moisture has been absorbed (about 6 minutes). Don't stir too much to allow the mushrooms to brown. (Use the same pan to precook the chicken later).
Add garlic: Add the crushed garlic, soy sauce, balsamic vinegar, pepper to taste and stir through. Cook for a few more seconds stirring all the time then remove from the heat and set aside for a few minutes to cool.Preheat the oven to 375F/190C/180fan/gas mark 5. You will need a large shallow dish or baking sheet. Cut chicken: Using a sharp knife cut a pocket into the side of each chicken breast (starting from the thickest part) about ¾ of the way through (as far as you can) being careful not to cut through the opposite side.
Tenderise chicken: Place the chicken breasts (cut side up) on top of a cutting board and cover with plastic wrap. Using a rolling pin or the bottom of a frying pan gently pound the thicker part of the meat a few times to tenderise and flatten it a little (be careful not to flatten it too much).
Season chicken: Sprinkle each chicken breast all over with salt, pepper, paprika and thyme. Rub the seasoning and herbs into the meat to make sure it's covered evenly.
Season chicken: Sprinkle each chicken breast all over with salt, pepper, paprika and thyme. Rub the seasoning and herbs into the meat to make sure it's covered evenly.
Fry chicken: Using the same pan you cooked the mushrooms in heat 1 tablespoon of oil. Next slide a large spatula underneath each stuffed chicken breast and carefully transfer to the pan (do this in batches if your pan isn't large enough). Fry over a medium heat for 2-3 minutes on each side until lightly browned (use a large spatula to carefully turn the chicken over).
Bake: Transfer the chicken breasts to your oven dish or baking sheet lined with parchment, pour over any juices from the pan and cook in the centre of the oven for 20 minutes (or until an instant thermometer inserted in the thickest part of the chicken registers 165F/74C).
Serve: Remove from the oven, 'rest' for 3-4 minutes then serve!