Preheat the oven to 425°F (220°C). Line a large sheet pan with parchment paper or lightly grease it.
Place the halved baby potatoes on the sheet pan. Drizzle with 1 tablespoon olive oil, sprinkle with half of the garlic powder, half of the dried oregano, salt, and pepper. Toss to coat evenly and spread out in a single layer.
Roast the potatoes in the preheated oven for 10 minutes.
Meanwhile, in a large bowl, combine the red bell pepper, yellow bell pepper, zucchini, and red onion. Drizzle with 2 tablespoons olive oil, remaining garlic powder, oregano, smoked paprika, salt, and pepper. Toss to coat evenly.
After the potatoes have roasted for 10 minutes, remove the sheet pan from the oven and add the mixed vegetables to the pan, spreading them out evenly around the potatoes.
Place the salmon fillets skin-side down on the sheet pan in the spaces between the vegetables. Season the salmon with salt and pepper. Lay lemon slices over each salmon fillet.
Return the sheet pan to the oven and roast for an additional 10 minutes, or until the salmon is cooked through and flakes easily with a fork, and the vegetables are tender.
Remove from the oven and sprinkle chopped fresh parsley over the salmon and vegetables before serving.