Place cucumber in small colander set over small bowl; sprinkle with ½ teaspoon salt. Drain 1 hour.
For tzatziki sauce, stir yogurt, cucumber, 1 tablespoon lemon juice, ½ teaspoon lemon peel, ½ teaspoon garlic and ¼ teaspoon salt in small bowl until combined. Cover and refrigerate until ready to use.
Combine remaining 2 tablespoons lemon juice, ½ teaspoon lemon peel, ½ teaspoon garlic, ¼ teaspoon salt and pepper in small bowl; mix well. Rub evenly onto salmon.
Heat nonstick grill pan over medium-high heat. Cook salmon 5 minutes per side or until fish begins to flake when tested with fork. Wrap 2 fillets in plastic wrap for salmon cakes; refrigerate. Serve remaining salmon with sauce.
