In a mixing bowl, whisk together the honey, Dijon mustard, whole grain mustard, olive oil, minced garlic, dried thyme, salt, and pepper until well combined.
Place the chicken breasts in a resealable plastic bag or a shallow dish, then pour the honey mustard marinade over the chicken, ensuring each piece is well coated. Seal the bag or cover the dish and marinate in the refrigerator for at least 30 minutes (or up to overnight for deeper flavor).
Transfer the marinated chicken breasts to a baking dish, reserving the marinade. Bake in the preheated oven for about 25-30 minutes, or until the chicken is cooked through and the internal temperature reaches 165°F (75°C).
While the chicken is baking, pour the reserved marinade into a small saucepan. Boil it for 5-7 minutes to create a thick, flavorful glaze.
After baking, brush the thickened glaze over the chicken breasts and return to the oven for an additional 5 minutes. Remove from the oven and let rest for a few minutes.
Slice the chicken and serve warm, drizzled with any remaining glaze and garnished with fresh chopped parsley.