Preheat the oven to 175 degrees Celsius (350 degrees Fahrenheit)
Generously season both sides of each chicken breast with salt and pepper. Heat the olive oil in a cast iron skillet and then add in the chicken breasts and sear for 3 minutes per side until golden in colour and then transfer the chicken to a plate.
Add the diced onion, and crushed garlic to the skillet and cook for approximately 4 minutes until the onions begin to soften.
In a bowl whisk together the mustard and honey. Add the chicken stock to the skillet and bring to a boil before adding in the honey mustard mixture and thyme. Let everything simmer for 3 to 4 minutes before returning the chicken breasts to the pan. Spoon some of the sauce over the chicken and then transfer the skillet to the oven. Bake for 30 minutes. Sprinkle the chicken with a bit more thyme before serving.