Combine fajita seasoning with salt and cayenne pepper and set aside.
In a wide bowl, whisk together olive oil, lime juice, orange juice, soy sauce, brown sugar, and minced garlic to make the marinade. Set aside 1 tablespoon of this marinade.
Add salmon fillets to the marinade and let them soak for 30 to 60 minutes at room temperature while you prepare the vegetables.
Heat oven to 400°F and cover a baking sheet with foil, spraying it lightly with cooking spray.
Place sliced onions and bell peppers on the baking sheet and season with 1 tablespoon of the fajita seasoning mix, salt, olive oil, and the reserved tablespoon of the marinade. Toss everything to coat well and spread out in an even layer.
Roast the vegetables for 5 to 10 minutes depending on your salmon thickness, shorter time if salmon is thick, longer if thin.
Remove the pan from oven, stir the vegetables, and push them to one side.
Place the marinated salmon fillets on the empty side and rub them well with the remaining fajita seasoning.
Return the pan to the oven and bake for 12 to 16 minutes, until the salmon is cooked through and flakes easily.
Set the oven to broil and cook salmon and vegetables for 1 to 2 minutes to get a nice char and golden color.
Drizzle the melted Cilantro Lime Butter over the fish.
Add salt and pepper to taste.
Serve the salmon and vegetables wrapped in tortillas or over rice or salad, topping with any combination of black beans, tomatoes, avocados, guacamole, sour cream, hot sauce, and extra lime juice as desired.