Whisk together the lemon juice, dill, salt, and pepper in a small bowl. Gradually whisk in the olive oil to combine.
Preheat the broiler. Line a large rimmed baking sheet with foil and place the salmon on the baking sheet. Drizzle with the olive oil and season with the salt and pepper. Broil the salmon for 6 to 8 minutes, until the fish is cooked through and flakes easily. Remove from the oven and, using two forks, break the fish up into bite-size pieces.
Meanwhile, bring a large pot of salted water to a boil. Add the asparagus and cook for 2 to 3 minutes, until crisp-tender. Transfer the asparagus to a bowl of ice water to stop the cooking process. Drain and set aside. Add the couscous to the boiling water, remove from the heat, and let sit for 5 to 6 minutes, until al dente. Drain the pasta.
Toss together the asparagus, couscous, salmon, red onion, and feta in a large bowl to combine. Drizzle the dressing over the salad and toss to coat.