Preheat oven to 425°F. Line a large, rimmed baking sheet with foil and spray with nonstick cooking spray. Spread the broccoli on the prepared pan, drizzle with the olive oil, and toss to coat. Season with about ¼ teaspoon kosher salt and some ground black pepper to taste.
Push the broccoli to the sides of the pan and arrange the salmon (skin-side down) in the center. In a small bowl, whisk together honey, mustard, soy sauce, garlic powder, ¼ teaspoon kosher salt, and ¼ teaspoon ground black pepper. Brush the glaze on top of each piece of salmon. Reserve the extra glaze, if desired.
Bake until the salmon flakes easily with a fork, about 14-15 minutes. For a crispier finish, place the pan under the broiler for the final 1-2 minutes of cooking (just keep an eye on it closely so that it doesn’t burn).
This step is optional, if you'd like to serve the salmon with the extra glaze: While the salmon is in the oven, simmer the remaining glaze in a small saucepan over medium heat on the stovetop (or in a microwave-safe bowl in the microwave) for 1-2 minutes, until the glaze thickens slightly.
Serve salmon and broccoli over rice, drizzle or brush with remaining glaze, and garnish with sliced green onions.