Place the sweet potatoes on a large rimmed sheet pan. Drizzle with olive oil and sprinkle with rosemary, salt, and pepper. Toss until sweet potatoes are coated. Spread the sweet potato rounds in an even layer on the pan. Place the pan in the oven for 12 minutes.
Meanwhile, combine the softened butter, garlic, honey, rosemary, salt, and pepper in a small bowl. Season the salmon fillets with salt and pepper.
Remove the sheet pan from the oven. Place the salmon fillets on top of the sweet potatoes. Spread the butter mixture evenly on salmon pieces. Place the lemon slices on top. Put the pan back in the oven and cook for 10 to 15 minutes or until the salmon is opaque throughout. The cook time will depend on the thickness of your salmon pieces. Check at 10 minutes to be safe.
Remove from the oven and lightly drizzle honey over the sweet potatoes and salmon. Serve immediately.