Cut cucumbers into 1.5-inch rounds. Using a small spoon or melon baller, scoop out the center of each round about ½ inch deep, leaving the bottom intact to form a cup.
In a medium bowl, flake the leftover salmon into bite-sized pieces with a fork.
Add Greek yogurt, Dijon mustard, lemon juice, dill, celery, salt, and pepper. Fold gently until combined — try not to break the salmon down too much.
If using, fold in the chopped leftover asparagus.
Taste and adjust lemon or salt as needed.
Spoon the salmon salad into the cucumber cups, mounding slightly.
Garnish with a pinch of fresh dill and a little lemon zest on top.
Serve immediately with lemon wedges, or chill for up to 2 hours before serving.