Preheat the oven to 425°F (220°C). Line a large sheet pan with parchment paper or aluminum foil.
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 Italian seasoning, salt, and 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 begin roasting, pat the salmon fillets dry with paper towels. Brush both sides of the salmon fillets with the remaining 1 tablespoon of olive oil and season with salt and pepper.
Remove the sheet pan from the oven after 10 minutes. Make space in the center of the pan and place the salmon fillets skin-side down.
Arrange lemon slices over the salmon fillets.
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.