Combine the fajita seasoning with salt and cayenne pepper in a bowl; set aside.
Prepare the marinade by whisking together olive oil, lime juice, orange juice, soy sauce, brown sugar, and minced garlic in a shallow dish. Reserve one tablespoon of the marinade for later use.
Place the salmon in the marinade, letting it soak for 30 to 60 minutes at room temperature while you slice the vegetables.
Heat the oven to 400°F and line a baking sheet with foil, spraying it lightly with cooking spray.
Arrange sliced onions and bell peppers on the baking sheet. Sprinkle with fajita seasoning, salt, drizzle with olive oil and the reserved marinade, then toss to coat evenly.
Roast the veggies for 5 to 10 minutes, adjusting time based on salmon thickness and how tender you want the vegetables.
Shift the vegetables to one side and place the salmon fillets on the pan. Rub the salmon with the remaining fajita seasoning.
Bake for 12 to 16 minutes or until the salmon flakes easily with a fork, depending on thickness.
Turn on the broiler and broil the salmon and vegetables for 1 to 2 minutes until slightly charred and golden.
Mix melted butter, lime juice, cilantro, and salt to create the Cilantro Lime Butter, and spoon it on top of the salmon.
Season with extra salt and pepper to taste before serving.
Serve the salmon and veggies in tortillas or over rice or salad, topped with any desired garnishes like black beans, tomatoes, avocados, guacamole, sour cream, hot sauce, and extra lime juice.
