Heat a non-stick pan over medium high heat and spray with cooking spray. Add the asparagus spears and whole walnuts. Cook, tossing occasionally until asparagus is starting to brown, 5 - 7 minutes. Remove from heat.
In a small jar, whisk together the sour cream, brown sugar, lemon juice, poppy seeds and salt.
In a large bowl, toss together the cooked asparagus and walnuts, cherries and feta cheese with the spring mix lettuce. Top with the dressing. I had some dressing leftover for another day.