2 tablespoons fresh lemon juice
2 tablespoons minced red onion
1 tablespoon olive oil
1 tablespoon drained capers, chopped
1 teaspoon chopped fresh thyme
1/2 teaspoon grated lemon peel
1 1 1/2-pound skinless salmon fillet (1 1/4 to 1 1/2 inches thick)
1 pound asparagus, trimmed
1 tablespoon extra-virgin olive oil
Whisk first 6 ingredients in small bowl to blend. Season sauce with salt and pepper. Preheat oven to 450°F. Cut three 1/2-inch-deep slits crosswise in top of salmon (as if dividing into 4 equal pieces but do not cut through). Arrange asparagus in even layer on rimmed baking sheet. Drizzle with oil and turn to coat. Sprinkle with salt and pepper. Place salmon atop asparagus; sprinkle with salt and pepper. Roast until salmon is just opaque in center, about 20 minutes.
Transfer asparagus and salmon to platter. Spoon sauce over salmon. Cut into 4 pieces along slits and serve.