Preheat your grill to medium-high heat.
Remove the backbone of the chicken using kitchen shears or a sharp knife. Open up the chicken and flatten it out, pressing down on the breastbone with your hands.
In a large bowl, whisk together the olive oil, Dijon mustard, soy sauce, lemon zest, minced garlic, chopped rosemary leaves, kosher salt, and black pepper.
Place the flattened chicken in the bowl with the marinade, making sure to coat the chicken thoroughly. Let the chicken marinate for at least 30 minutes, or up to overnight in the refrigerator.
Once the grill is hot, remove the chicken from the marinade and place it on the grill, skin-side down. Grill the chicken for 10-12 minutes per side, or until the internal temperature reaches 165°F.
Remove the chicken from the grill and let it rest for 5-10 minutes before serving.