About 30 minutes before roasting place the salmon fillet on a rimmed baking sheet and pat dry. Sprinkle 1-½ teaspoons of the kosher salt and ½ teaspoon of the pepper over the flesh of the salmon. Let the salmon rest for about 20 to 30 minutes to take the chill off before roasting.
Lightly spray a large ovenproof sauté pan with cooking spray. You’ll want to use a pan large enough to hold the salmon fillet when it's folded in half. Melt the butter with the olive oil in the pan over medium heat.
Add the sliced fennel bulb and the onion. Sauté for about 5 minutes then stir in the orange zest and the orange juice. You should have about 1 teaspoon orange zest and ½ cup orange juice.
Sprinkle with the remaining 1 teaspoon of kosher salt and ½ teaspoon of pepper. Check for seasoning, adding more kosher salt and pepper if needed.
Toss in the fennel fronds, the parsley and the thyme. Cook for an additional 7 to 10 minutes or until the onions and fennel are both very tender and the herbs are wilted.
Take about half of the onions, fennel and herb mixture and spread it on the large half of the salmon fillet. Top with slices of orange and lemon. Fold the tail end of the salmon up over the filling.
Place the salmon fillet into the pan setting it on top of the remaining onion and fennel mixture.
Roast the salmon in a 450-degree oven for 35 minutes.
Serve the salmon topped with some of the roasted fennel and onion along with an extra squeeze of fresh lemon or orange juice. Wonderful hot or at room temperature.