Preheat oven to 425°F (218°C). Pat chicken breasts dry with paper towels. If breasts are significantly thicker at one end, use a meat mallet to gently pound to an even thickness of approximately ¾ inch. Season both sides with salt and pepper.
In a small bowl, combine Dijon mustard, honey, minced garlic, thyme leaves, lemon zest, lemon juice, and cayenne pepper (if using). Mix thoroughly until well incorporated into a smooth paste.
Line a rimmed baking sheet with parchment paper or aluminum foil. Brush the surface with olive oil to prevent sticking.
Place chicken breasts on the prepared baking sheet. Using a pastry brush or the back of a spoon, apply the Dijon mixture generously to both sides of each breast, ensuring even coverage.
Place the baking sheet on the middle rack of the preheated oven. Bake for 5 minutes at 425°F (218°C).
Without opening the oven door, reduce temperature to 375°F (190°C) and continue baking for 15-18 minutes, or until the internal temperature reaches exactly 165°F (74°C) at the thickest part.
Remove chicken from the oven and transfer to a cutting board. Tent loosely with foil and allow to rest for 5-7 minutes before slicing to allow juices to redistribute.