1. Preheat the oven to 375°F or 190°C.

2. In a large skillet melt the butter over medium heat then add the mushrooms, garlic, and thyme.

3. Cook until the mushrooms release their liquid and start to brown, this should take about 5 minutes.

4. Add the white wine to the skillet scraping up any browned bits from the bottom of the pan and cook until the liquid has almost completely evaporated, this should take about 3 minutes.

5. Season the mixture with salt and pepper to taste.

6. In a large bowl whisk together the heavy cream and green peppercorns in brine until well combined.

7. Add the chicken breasts to the bowl making sure they are fully coated in the cream mixture.

8. Grease a 9x13 inch or 23x33 cm baking dish and arrange half of the mushroom mixture in the bottom of the dish.

9. Top with the chicken breasts then cover with the remaining mushroom mixture.

10. Sprinkle the Dijon mustard over the top of the gratin.

11. Bake in the preheated oven for 35-40 minutes or until the chicken is cooked through and the top is golden brown.

12. Remove the dish from the oven and let it rest for 10 minutes before serving.

13. Slice the chicken into thick strips and serve with the creamy mushroom sauce spooned over the top.