Prepare the chicken by patting it dry with paper towels. If the breasts are thick, place them between plastic wrap and gently pound to an even 3/4-inch thickness. Season both sides generously with salt, pepper, and garlic powder.

Heat olive oil and 1 tablespoon butter in a large skillet over medium-high heat. Once the butter is melted and the pan is hot, add the chicken breasts. Cook for 6-7 minutes without moving them, until a golden-brown crust forms on the bottom.

Flip the chicken breasts and cook for another 5-6 minutes until the internal temperature reaches 165°F. Transfer the chicken to a plate and tent loosely with foil to keep warm.

Reduce heat to medium and add the remaining tablespoon of butter to the same skillet. Add minced garlic and sauté for 30-60 seconds until fragrant but not browned.

Pour in the chicken broth (or wine) and use a wooden spoon to scrape up any browned bits from the bottom of the pan. Let it simmer for 2-3 minutes until slightly reduced.

Reduce heat to medium-low and stir in the heavy cream, Dijon mustard, whole grain mustard (if using), thyme, salt, and pepper. Whisk until smooth and well combined.

Let the sauce simmer gently for 3-4 minutes, stirring occasionally, until it thickens slightly and coats the back of a spoon. Taste and adjust seasonings as needed, adding lemon juice for brightness if desired.

Return the chicken breasts to the skillet, spooning sauce over them. Let them warm through for 2-3 minutes, then remove from heat.

Garnish with freshly chopped parsley and serve immediately with your choice of sides. Spoon extra sauce over the chicken and sides just before serving.