Preheat the oven to 425°F (220°C).
In a large bowl, toss the sweet potato cubes with 1 tablespoon of olive oil, garlic powder, dried thyme, 1/4 teaspoon salt, and 1/8 teaspoon black pepper until evenly coated.
Spread the sweet potatoes in a single layer on a large sheet pan.
Roast the sweet potatoes in the preheated oven for 10 minutes.
While the sweet potatoes begin roasting, pat the salmon fillets dry with paper towels.
Rub the salmon fillets with the remaining 1 tablespoon olive oil, minced garlic, 1/4 teaspoon salt, and 1/8 teaspoon black pepper.
After the sweet potatoes have roasted for 10 minutes, remove the sheet pan from the oven and push the sweet potatoes to one side.
Place the salmon fillets skin-side down on the empty side of the sheet pan.
Arrange lemon slices on top of the salmon fillets.
Return the sheet pan to the oven and roast for an additional 15 minutes, or until the salmon is cooked through and flakes easily with a fork and the sweet potatoes are tender.
Remove from the oven and sprinkle fresh parsley over the salmon and sweet potatoes before serving.