Preheat oven to 400°F (200°C) and line a large rimmed sheet pan with parchment paper or aluminum foil.
In a large bowl, toss the halved Brussels sprouts with 2 tablespoons of olive oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper.
Spread the Brussels sprouts in a single layer on the prepared sheet pan and roast for 10 minutes.
While sprouts roast, pat salmon fillets dry and season with remaining olive oil, salt, pepper, and garlic powder.
Move the Brussels sprouts to the outer edges of the pan and place salmon fillets in the center.
Top salmon fillets with lemon slices and thyme leaves.
Return the pan to the oven and roast for an additional 10 to 12 minutes until the salmon reaches an internal temperature of 145°F (63°C) and sprouts are tender.
Serve immediately.