Start by making the mushroom mixture. Chop the mushrooms roughly and pulse until crumbly. Be careful not to puree the mixture.
In a large pan melt 1 tablespoon of butter and fry the mushrooms with the seasoning over a fairly high heat until all the moisture has been absorbed (about 6 minutes). Set aside to cool for a few minutes.
Preheat the oven to 375 F/ 190 C/ gas mark 5. Lightly grease the bottom of a medium size casserole dish with cooking spray and set aside.
To make the sauce in a saucepan melt 1.5 teaspoon of butter, add the flour and whisk until smooth. Pour in the milk and continue cooking and whisking until the sauce starts bubbling up. Remove from the heat, whisk in 4 tablespoons of the parmesan and season to taste. Set aside.
Cut each chicken breast lengthways at a slight angle and cut a pocket into the side of each chicken piece being careful not to cut through the opposite side. Set aside.
Place the mushroom mixture in a bowl and add the ricotta and breadcrumbs. Stir thoroughly and adjust the seasoning if needed.
Stuff each chicken piece with the mushroom mixture and arrange inside the casserole dish.
Spoon the sauce over the top of each chicken piece (if the sauce has thickened too much add a drop of milk to loosen it), scatter the remaining parmesan over the whole thing and bake in the centre of the oven for 30-35 minutes or until the chicken is fully cooked. Remove from the oven and serve!
Pulse the mushrooms for a few seconds only until they become crumbly. Do NOT overprocess and puree the mixture. You can chop the mushrooms by hand but it's a laborious process - the food processor does it in seconds.
You can make the mushroom mixture ahead and once cooled refrigerate until you are ready to make the recipe.
Seasoning: Be generous with the pepper.
Cooking time: This will depend on the size of your chicken pieces. Try not to overcook the chicken as it may become a little tough.
You can make the sauce ahead, cover and refrigerate (once completely cooled) overnight. The sauce may thicken as it cools so you may want to add a drop of milk to loosen it before adding into the dish.