Step 1

Rinse the frisée well to remove any grit.

Step 2

Pat tuna dry with paper towels. Sprinkle all over with 1 teaspoon salt and ¾ teaspoon pepper. Combine onion and lemon juice in large bowl; set aside. Bring 2 quarts water to boil in large saucepan over medium-high heat. Add potatoes and 2 tablespoons salt; return to boil and cook until tender, 8 to 10 minutes. Drain potatoes, then spread out on large plate.

Step 3

Meanwhile, heat oil in medium saucepan over medium-low heat until shimmering. Add tuna and cook, covered, until reddish pink at center when checked with tip of paring knife and registering 120 to 125 degrees (for medium-rare), 8 to 10 minutes, flipping tuna halfway through cooking. (Adjust heat as necessary to maintain small bubbles around edges of tuna.) Transfer tuna to plate and flake into large pieces with fork.

Step 4

Mix ¼ cup oil from saucepan, lemon zest, remaining ½ teaspoon salt, and remaining ¼ teaspoon pepper into onion mixture. Add potatoes, frisée, orange, parsley, and olives and toss to combine. Season with salt and pepper to taste. Transfer salad to platter and sprinkle tuna on top. Serve.