Place chicken between two sheets of plastic wrap and pound to an even 1/2-inch thickness. Pat completely dry with paper towels. Season evenly with salt, pepper, and garlic powder on both sides.
Heat a 12-inch stainless steel skillet over medium-high heat until a droplet of water dances across the surface (approximately 375°F).
Add oil to the skillet and swirl to coat. Carefully place chicken breasts in the hot skillet without crowding. Cook undisturbed for 5-6 minutes until deeply golden brown. Flip and cook 4-5 minutes on second side until internal temperature reaches 160°F. Transfer to a clean plate and tent loosely with foil.
Reduce heat to medium-low. Add butter to the skillet. Once melted, add minced shallots and cook for 2 minutes until translucent but not browned. Add garlic and cook 30 seconds until fragrant.
Add white wine and scrape up all browned bits from the pan bottom with a wooden spoon. Simmer until wine is reduced by half, about 2 minutes.
Whisk in both Dijon and whole grain mustards until fully integrated, about 30 seconds.
Remove pan from heat momentarily. Slowly pour in heavy cream while whisking constantly. Return to low heat and simmer gently for 3-4 minutes until sauce thickens slightly.
Stir in thyme leaves, lemon zest, and lemon juice. Taste and adjust seasoning with salt and pepper as needed.
Place chicken and any accumulated juices back into the skillet. Spoon sauce over chicken and simmer gently for 1-2 minutes to reheat chicken and allow flavors to meld.
Garnish with fresh tarragon if using. Serve with sauce spooned over chicken.