Rinse the fish thoroughly under cold running water. Remove the entrails, if any. Spread the cavity with your fingers and wash it from the inside with a good stream. Pat dry each fish using a kitchen paper and set aside.
In a large bowl, combine olive oil, orange juice, orange extract, chili flakes, dried thyme, garlic, and salt. Mix until combined and submerge fish in this mixture. Coat well and refrigerate for 30 minutes before grilling.
Preheat the grill to a medium-low heat. Drain the fish and place on a grill rack.
Grill for 4-5 minutes on each side or until nice and crispy. Generously brush the fish with the marinade while grilling. About three tablespoons will be enough. This will keep the moisture and juiciness. Remove from the grill and sprinkle with rosemary. Garnish with grilled orange slices.
Serve with low-carb veggies and enjoy while it's still hot.