In an 8-inch cast iron skillet, add 2 T olive oil. Place the skillet in the oven while it preheats to 425 degrees.
In a medium bowl, add the garlic, Dijon mustard, Champagne vinegar, soy sauce, 1/4 C olive oil and thyme leaves. Whisk together until combined.
Remove the skillet from the oven and place the chicken breasts, skin side up, in the skillet.
Season generously with ground pepper. Pour the mustard mixture over the chicken breasts.
Bake for 20 minutes. Remove from the oven and baste with the skillet juices.
Return to oven and bake for another 10 - 15 minutes, until chicken is cooked through.
Let rest, at room temperature, for 5 minutes before serving.
I highly recommend serving this chicken over steamed rice.