Preheat and Prepare Spices: Position a rack in the center of your oven and preheat to 400°F (200°C). In a small bowl, whisk together the onion powder, garlic powder, smoked paprika, ¼ teaspoon of the salt, ⅛ teaspoon of black pepper, and the optional cayenne pepper.
Season the Vegetables: In a large mixing bowl, combine the diced sweet potatoes and broccoli florets. Drizzle with the avocado oil and pour over the unsalted vegetable broth. Sprinkle with the remaining ⅛ teaspoon of salt and ⅛ teaspoon of black pepper. Toss everything thoroughly to ensure an even coating.
Roast Vegetables: Spread the seasoned vegetables in a single, even layer on a large baking pan. Transfer to the preheated oven and roast for 20 minutes, stirring halfway through, until the sweet potatoes begin to tenderize.
Prepare Salmon & Sauce: While the vegetables are roasting, rub each salmon fillet on all sides with the prepared spice mixture. In a separate medium bowl, create the sauce by whisking together the fat-free Greek yogurt, lemon zest, lemon juice, fresh dill, and chives until smooth.
Combine and Finish Roasting: Remove the pan from the oven. Carefully clear six spaces among the vegetables and arrange the salmon fillets on the pan. Return the pan to the oven and roast for an additional 10-12 minutes, or until the salmon's internal temperature reaches 145°F (63°C).
Serve: Divide the vegetables and salmon among six plates. Drizzle generously with the dill-yogurt sauce and serve immediately.