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Boil the kettle, tip the mushrooms into a small bowl, pour over boiling water to just cover, then leave to soak for 15 mins. Drain and squeeze out the mushrooms (save and freeze the liquid to make a base for a gravy) and roughly chop.
Heat the oil in a frying pan and gently cook the shallots and garlic for 3 mins. Turn up the heat, throw in the mushrooms and cook for a few mins more. Add the spinach, turn the heat up to high and cook until wilted and all the liquid has evaporated. Tip into a bowl and leave to cool. Beat in the cheeses and season with loads of pepper and a little salt. Set aside.
Take the chicken breasts and remove the small fillet from the back. Cut a slit into the side of each chicken breast then stuff each one with a quarter of the spinach mixture, closing it with the cut-off fillet. Carefully wrap each stuffed chicken breast in a slice of prosciutto and chill until needed. Can be prepared up to a day ahead.
To cook, heat oven to 200C/180C fan/gas 6. Line up the chicken on a baking tray and roast for 20-25 mins until the prosciutto is frazzled at the edges, the cheese is oozing out and the chicken is cooked through.