Preheat the oven to 400°F (200°C). Line a large sheet pan with parchment paper or lightly grease it.
In a large bowl, combine the halved baby potatoes, red bell pepper pieces, yellow bell pepper pieces, zucchini slices, and red onion wedges.
Drizzle 2 tablespoons of olive oil over the vegetables. Sprinkle with garlic powder, dried thyme, dried oregano, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Toss well to coat evenly.
Spread the seasoned vegetables in a single layer on the prepared sheet pan, leaving space in the center for the salmon fillets.
Place the sheet pan in the oven and roast the vegetables for 10 minutes.
While the vegetables start roasting, pat the salmon fillets dry with paper towels. Brush both sides of each fillet with the remaining 1 tablespoon olive oil. Season both sides with the remaining 1/2 teaspoon salt and 1/4 teaspoon black pepper.
After the vegetables have roasted for 10 minutes, remove the sheet pan from the oven. Arrange the salmon fillets skin-side down in the center of the sheet pan.
Place 1-2 lemon slices on top of 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 the sheet pan from the oven. Sprinkle the chopped fresh parsley over the salmon and vegetables.
Serve immediately, garnished with additional lemon slices if desired.