Rinse the fish under cold running water. Pat dry with paper towels.
In a shallow dish, mix the olive oil, honey, mustard, garlic, lemon juice, and chili powder, if using. Place the fish in the mixture, turning to coat it with the marinade. Cover and marinate for 2 to 3 hours in the refrigerator or, if pressed for time, for 30 minutes at room temperature.
Preheat the oven to 375°F.
Transfer the fish to a baking dish. Bake for about 12 minutes, or until the flesh flakes nicely with a fork. Season with salt.
Meanwhile, core the apples and cut each into 8 wedges. Heat the butter in a large skillet over medium-high heat. Add the apples and rosemary, reduce the heat to medium, and cook for 5 minutes, until tender. Remove from the heat and discard the rosemary.
Serve the fish with the apples, garnished with mint, if desired.
Reserve the marinade. After 8 minutes of baking, pour the marinade over the salmon. Serve the salmon with the pan juices spooned over it.