Thaw tuna steaks, if frozen. Rinse quinoa well in a fine-mesh sieve. Bring the water and 1/4 teaspoon salt to boiling. Add quinoa; reduce heat. Simmer, covered, about 15 minutes or until most of the water is absorbed and quinoa is tender. Drain if necessary. Spread quinoa in a shallow baking pan to cool slightly.
In a bowl combine quinoa, tomato, cucumber, feta cheese, and red onion.
For dressing, in a screw-top jar combine olive oil, lemon juice, honey, snipped oregano, and 1/4 teaspoon pepper. Cover and shake well. Add dressing to quinoa mixture and toss to combine.
Lightly coat an unheated grill pan or large skillet with cooking spray. Preheat grill pan over medium-high heat. Sprinkle tuna steaks with 1/8 teaspoon salt and 1/8 teaspoon pepper. Cook tuna on grill pan for 4 to 6 minutes per 1/2-inch thickness of fish, turning once, or until browned on both sides, fish begins to flake when tested with a fork, and center is just pink. Thinly slice tuna steaks. Serve tuna on top of quinoa salad on six serving plates. Garnish with fresh oregano leaves.