Preheat oven to 400° F.

Pat chicken breasts dry with paper towels. Season both sides with salt and pepper.

Heat 2 tablespoons oil in a cast-iron skillet or large saute pan over medium-high heat. Add chicken breasts. (You may need to do this in two batches. You do not want to overcrowd the pan or the chicken will not brown.) Brown chicken on both sides and transfer to plate.

Add remaining oil and add onions to the pan. Cook until the onions begin to soften and brown. Add garlic and cook for another minute. If using, add wine, and scrape the brown bits from the bottom of the pan.

Mix together both mustards, honey, rosemary, and thyme leaves. Season with salt and pepper. Set aside.

Spread the onion mixture evenly in the pan. Place the chicken over the onions. Pour the honey mustard over the chicken pieces.

Add the pan to the oven and cook chicken until cooked through and reaches an internal temperature of 165° F, about 15 to 20 minutes.

Spoon the pan sauce over the chicken breast when serving.