Preheat oven to 425°. Cut each onion into 3 (¾-inch-thick) slices. Line a 15×10-in. rimmed baking pan with foil. Arrange onion slices in prepared pan. Brush tops of onion slices with 1 tablespoon olive oil. Sprinkle with ½ teaspoon salt and pepper. Roast for 30 minutes.

Pat salmon dry with paper towels. Place in baking pan with onions, tucking under thin edges of salmon if necessary. Brush salmon with lime juice; sprinkle with chipotle powder and ¼ teaspoon salt. Roast for 6 to 9 minutes or until salmon flakes easily with a fork (145°) and onions begin to brown.

Combine walnuts, vinegar, Fresno chile, remaining 1 tablespoon olive oil, garlic, and remaining ¼ teaspoon salt in a small bowl; set aside.

Combine arugula and cucumber in a large bowl. Add about half the walnut mixture and all of its liquid; toss until combined.

To serve, arrange arugula mixture and onion slices on a large serving platter. Flake salmon into large pieces; arrange on salad. Spoon remaining walnut mixture on top.