Preheat the oven to 400°F (200°C).
Line a large sheet pan with parchment paper or lightly grease it with 1 tablespoon of olive oil.
In a large bowl, combine the halved baby potatoes, sliced red and yellow bell peppers, zucchini slices, and red onion wedges.
Drizzle the vegetables with 2 tablespoons of olive oil, then sprinkle with garlic powder, smoked paprika, dried thyme, salt, and black pepper. Toss well to evenly coat all the vegetables.
Spread the seasoned vegetables in an even layer on the prepared sheet pan.
Place the sheet pan in the oven and roast the vegetables for 10 minutes.
Remove the sheet pan from the oven and push the vegetables to the sides to make space in the center for the salmon fillets.
Place the salmon fillets skin-side down in the center of the sheet pan. Season the salmon with salt and black pepper.
Top each salmon fillet with a lemon slice.
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 and sprinkle the chopped fresh parsley over the salmon and vegetables.
Serve the salmon fillets with the roasted vegetables immediately.