Begin by preheating your oven to 400°F (200°C). Lightly coat your ceramic baking dish or cast iron skillet with 1 tbsp (15ml) of olive oil.

Place the chicken breasts flat on a clean cutting board. Using a sharp paring knife, carefully slice a horizontal pocket into the thickest side of each breast, stopping 1/2-inch (1.25cm) before reaching the opposite side.

Pat the outside of the chicken breasts completely dry with a paper towel. Season the chicken on all sides, as well as lightly inside the pockets, with the kosher salt and black pepper.

Divide the Boursin cheese into four equal portions. Gently press one portion deep into the pocket of each chicken breast. Secure with a toothpick if desired.

In a small mixing bowl, combine the panko breadcrumbs, finely chopped fresh parsley, and finely chopped fresh rosemary. Pour the melted butter over the mixture and toss with a fork until damp.

Smear a thin layer of Dijon mustard across the top of each stuffed chicken breast. Generously mound the buttery panko mixture onto the mustard, pressing down firmly to adhere.

Transfer the chicken to the prepared baking dish. Bake in the preheated oven for 25-30 minutes until the crust is deep golden-brown and the juices sizzle.

Verify the internal temperature has reached 165°F (74°C). Allow the chicken to rest for 5 minutes. Garnish with fresh sprigs of rosemary and serve.