4 (6 ounce) skinless salmon fillets
1 tablespoon salt-free garlic-and-herb seasoning blend (such as Mrs. Dash)
¾ teaspoon kosher salt, divided
3 tablespoons finely chopped red onion (from 1 small onion)
3 tablespoons olive oil
2 tablespoons fresh lemon juice (from 1 small lemon)
1 teaspoon Dijon mustard
6 cups chopped romaine lettuce hearts (from 2 hearts) (from about 12 ounces whole lettuce)
2 cups halved cherry tomatoes
2 (7 ounce) Persian cucumbers, sliced (1 1/2 cups)
1 tablespoon chopped fresh basil
1 tablespoon chopped fresh dill
1 ounce feta cheese, crumbled (about 1/4 cup)
2 tablespoons chopped pistachios