Preheat the oven to 400°F (200°C).
Line a baking sheet with parchment paper or lightly oil it.
Place the salmon fillets skin side down on the prepared sheet.
Drizzle with olive oil, then rub with minced garlic, oregano, salt, and pepper.
Arrange lemon slices, onion, tomatoes, and olives over and around the salmon.
Bake for 20–25 minutes, or until the fish flakes easily with a fork.
Remove from the oven, garnish with fresh parsley, and serve warm.