In large saute pan or skillet, heat the Olive Oil over medium high heat.
Add Chicken and cook until starting to turn golden 8 - 10 min.
Toss in Mushrooms and minced Garlic and cook for a further 3 - 5 min. Stirring occasionally.
Now add the Leeks and give it a stir, Turn heat down to just under medium and cover for an additional 5 min.
Slowly sprinkle over the All Purpose Flour (do NOT dump it all in at once) and stir as you add . Stir in well.
Now you can add the Mustard, Thyme, Salt and Pepper and give it a quick stir before continuing to add in the Chicken Broth, Balsamic Vinegar and the White Wine (or Water). Mix well and simmer for a couple of minutes.
Take off the heat and now add in the Whipping Cream. Taste and adjust taste if needed. Give it a quick stir, transfer to oven-proof casserole dish (6 cup dish) and set aside.
Preheat oven to 400F.