Step 1

Arrange a rack in the middle of the oven and heat to 425°F. While the oven heats, marinate the salmon and soak the onions (instructions below).

Step 2

Whisk the olive oil, vinegar, lemon juice, garlic, oregano, salt, and pepper together in large bowl. Transfer 3 tablespoons of the vinaigrette to a baking dish large enough to hold all the pieces of salmon in one layer. Add the salmon, turning them over gently a few times to evenly coat in the vinaigrette. Cover and refrigerate. Place the onion and water in a small bowl and set aside for 10 minutes to make the onion less potent. Drain and discard the liquid.

Step 3

Uncover the salmon and roast until it is cooked through and flakes easily, 8 to 12 minutes. An instant-read thermometer into the middle of the thickest fillet should register 120°F to 130°F for medium-rare, or 135°F to 145°F if you prefer it more well-done. Total cook time will depend on the thickness of salmon, based on the thickest part of the fillet. Meanwhile, compose the salad.

Step 4

Add the lettuce, tomato, cucumber, olives, and drained red onion to the bowl of vinaigrette and toss to evenly coat. Divide between 4 plates or shallow bowls. When the salmon is ready, place 1 fillet on top of each salad. Sprinkle with the feta and serve immediately.