Preheat oven to 400 degrees and spray a large baking sheet with cooking spray or line with foil.
Scatter the baking sheet with the onions, tomatoes and olives then top with the salmon filet. Place lemon slices on top of the salmon in a single layer. Drizzle with olive oil, salt, pepper and oregano then lightly toss the veggies to coat.
Bake until fish flakes easily with a fork, 16-18 minutes.