In a medium mixing bowl, add 1/2 teaspoon thyme, onion powder and flour.
Cut the whole chicken breast in half and pound it to an even thickness with the bottom of pan or a meat mallet. If you use the pre-cut chicken breast thin fillets, skip this part.
Season the chicken with 1/4 teaspoon salt. Dredge it to the seasoned flour and shake off the excess flour.
Melt butter in a skillet pan over medium-high heat. Add chicken, then sear it for about 1-2 minutes per side or until golden. Remove to a plate.
Using the same skillet (do not wash), add mushrooms and 1 teaspoon fresh thyme (or use more according to your liking). Cook until mushrooms are brown (for about 4-5 minutes).
Add garlic and cook until fragrant, for about 30 seconds.
Stir in dry white wine, chicken stock and heavy cream. Bring to a boil, stirring occasionally for about 2-3 minutes. Season with salt and peppercorn (or black pepper) to taste.
Return the chicken back to the pan. Keep it uncovered and reduce the heat to a medium-low. Simmer until the chicken is fully cooked and the sauce has thickened for about 5 minutes.
Remove the chicken and mushrooms from the heat. Serve immediately with steamed veggies, pasta or rice.