Preheat the oven to 400°F (200°C). Line a baking sheet or shallow baking dish with parchment for easy cleanup.

Pat the salmon dry with paper towels and place it skin-side down (if using skin-on) on the prepared pan. Let it sit at room temperature for 10–15 minutes.

Toss the thinly sliced red onion and fresh spinach with 1 tablespoon olive oil and a light pinch of salt to soften the spinach slightly.

Chop the sun-dried tomatoes if needed.

Drizzle the remaining olive oil over the salmon and sprinkle garlic powder, dried oregano, salt, and pepper across the fillet.

Spread the onion and spinach mixture over the salmon. Scatter chopped sun-dried tomatoes and crumbled feta on top.

Bake in the preheated oven for 12–18 minutes, depending on the thickness of the fillet.

Optional: switch to broil for 1–2 minutes to brown the feta and tomatoes.

Remove from the oven, let rest for 2–3 minutes, then garnish with chopped fresh parsley.