Preheat the oven to 425 degrees F (220 degrees C). Line a 10x15-inch baking pan with aluminum foil.
Arrange broccoli in a single layer on the prepared pan. Drizzle with 2 teaspoons oil. Sprinkle with garlic, thyme, oregano, and 1/8 teaspoon each salt and pepper.
Roast in the preheated oven for 10 minutes.
Drizzle the salmon with the remaining 1 teaspoon of oil and the remaining salt and pepper. Top with lemon slices. Add to the pan. Roast until vegetables are tender and fish flakes easily when tested with a fork, 12 to 15 minutes.