Butterfly the chicken breasts by slicing them horizontally through the centre to create two even cutlets. Lightly flatten if needed to ensure even thickness.
In a bowl, whisk together the Dijon mustard, honey, olive oil, orange juice, orange zest, garlic, rosemary, smoked paprika, salt, pepper, and chilli flakes (if using).
Add the chicken to the bowl and coat thoroughly in the marinade. Cover and refrigerate for at least 30 minutes, or up to overnight for best results.
Preheat the oven to 400°F. Line a baking tray with baking paper.
Arrange the chicken pieces in a single layer on the tray. Spoon over any remaining marinade. Bake for 20-25 minutes, or until the chicken is cooked through and lightly golden. For a more caramelised top, place under a hot grill for 2–3 minutes at the end of cooking.
Rest for 5 minutes before slicing. Spoon over any tray juices before serving.