Slice chicken breasts lengthwise, salt and pepper and brown on both sides in olive oil on medium heat until golden. Remove to a plate. (At this point chicken will not be cooked all the way through.)

To the same pan add the mushrooms together with chopped rosemary, salt and pepper and brown the mushrooms over medium heat for 2-3 minutes, add the chopped shallots and minced garlic and cook for 1-2 minutes longer. Add a heaped tablespoon of flour and stir to combine. Then add the white wine and scrape the bottom of the pan with a spatula to release the brown bits into the liquid. Pour in chicken stock, double/heavy cream, add 1 tbsp of mustard and stir to combine.

Then add the chicken breasts together with the juices they released back to the pan, and bring the sauce to a boil, then lower the heat to low and let it simmer for 10 minutes. Taste the sauce and add more salt and pepper if necessary.