Preheat your oven to 400°F and line a large rimmed baking sheet with parchment paper for easy cleanup.
Season the chicken breast evenly on both sides with the garlic powder, sea salt, and black pepper.
Place the seasoned chicken on the baking sheet and roast for 20 to 25 minutes, or until the internal temperature reaches 165°F.
While the chicken cooks, trim the woody ends off the asparagus spears and toss them with half of the olive oil.
Add the asparagus to the baking sheet during the final 12 minutes of the chicken's roasting time, cooking until tender-crisp.
In a small mixing bowl, whisk together the lemon juice, Dijon mustard, and the remaining olive oil to create the vinaigrette.
Once cooked, remove the chicken from the oven, let it rest for 5 minutes, and then slice it into thin strips.
Place the arugula in a large serving bowl and top with the roasted asparagus and sliced chicken breast.
Sprinkle the crumbled feta cheese and walnuts over the top, then drizzle with the lemon vinaigrette before serving.