Wash the fresh spinach thoroughly and dry it. Mince the garlic cloves and zest the lemon.
In a medium skillet, heat 1 tablespoon of olive oil over medium heat. Sauté the minced garlic for 1 minute until fragrant, then add the spinach and cook for 2-3 minutes until wilted. Season with salt, pepper, and oregano.
Preheat the oven to 400°F (200°C). Line a baking dish with parchment paper or grease it. Place the salmon fillets skin-side down, drizzle with olive oil, lemon juice, zest, salt, and pepper.
Distribute the sautéed spinach over the salmon fillets and sprinkle with Parmesan cheese if using.
Bake the salmon for 12-15 minutes until it flakes easily with a fork. Let it rest for a few minutes before serving.