Place a medium sized pot with a lid (I like to use a dutch oven as it retains heat well) on the stovetop on a medium high heat.
Slice the chicken breast fillets in half horizontally so that you have four flatter fillets. Season with salt, pepper and the Italian herb mix.
Add the olive oil to the pot and fry the chicken breasts in batches for a few minutes a side until just golden brown. Remove and set aside, covered.
Lower the heat to medium and add the butter, garlic and sage leaves to the pot. Stir until the butter is starting to brown.
Add the mushrooms, sundried tomatoes and their vinaigrette and the white wine. Stir briefly and cover immediately. Cook, covered, on medium to high heat for one minute and then take off the heat and rest for 15 minutes without removing the lid (this will steam the mushrooms and trap the liquid from the mushrooms for the sauce).
Place the pot on a low heat and stir in the mascarpone, spinach and grated parmesan. Add the chicken breasts back to the pot and cover for a few minutes to rewarm the chicken.
Season with freshly ground black pepper (this will need very little extra salt because of the sundried tomatoes) and serve (see note below for recommended sides).