Preheat the oven to 425°F (220°C).
Toss the halved potatoes with olive oil, salt, pepper, garlic powder, and rosemary. Arrange cut-side down on a preheated baking sheet and roast for 15 minutes.
While the potatoes roast, prepare the marinade by whisking together the lemon zest, lemon juice, olive oil, garlic, salt, pepper, Dijon mustard (if using), and dill. Place salmon in a shallow dish or zip-top bag and coat with marinade. Let sit for 10–15 minutes.
Remove the baking sheet from the oven. Flip the potatoes. Add broccoli to one side of the pan, tossing it with a little olive oil, lemon juice, and salt. Place the marinated salmon fillets in the center, skin-side down. Drizzle any leftover marinade over the top.
Return the tray to the oven and roast for an additional 12–15 minutes, or until the salmon flakes easily, the broccoli is browned at the edges, and the potatoes are golden and crispy.
Drizzle with olive oil, squeeze more fresh lemon juice on top, and garnish with fresh herbs. Serve immediately.