Thaw tuna or salmon and potatoes if frozen. Set fish aside. In a very large skillet heat the 2 tablespoons olive oil over medium heat. Add potatoes, onion, dill, cumin, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Spread evenly in skillet. Cook over medium-high heat for 20 to 25 minutes or until potatoes are golden brown and slightly crisp, stirring occasionally. Transfer to a medium bowl; cover and keep warm.
In the same skillet, heat 1 tablespoon olive oil over medium heat. Lightly sprinkle tuna with salt and pepper. Add tuna to skillet; cook for 4 minutes. Carefully turn tuna. Reduce heat to medium-low; cook for 2 to 4 minutes more or until fish flakes easily when tested with a fork (tuna should be slightly pink in the center). Remove tuna from skillet; keep warm. Add greens to skillet. Cook over medium-low heat, tossing frequently, for 5 to 7 minutes or until tender and beginning to wilt if using Swiss chard or kale; or 1 to 2 minutes or beginning to wilt if using spinach (add more oil to skillet during cooking if necessary).
Divide wilted greens among 4 serving plates. Top with potato mixture, tuna steaks, and, if desired, Caramelized Red Onion. Garnish with dill sprigs if desired. Pass salad dressing. Makes 4 servings.
In a small bowl stir together sour cream, mustard, honey, and salt. Stir in the 1/4 cup milk until smooth. If necessary, add additional milk, 1 teaspoon at a time, to make desired consistency. Makes about 1-1/4 cups.
Caramelized Red Onion
Heat oil in a medium skillet over medium heat. Add onion. Turn heat to medium-low. Cook onion for 8 to 10 minutes or until golden brown, stirring occasionally.