Toss the sliced cucumbers with 1/2 tsp salt in a colander and let drain for 30 mins, stirring occasionally. Rinse the cucumbers, then wrap in a clean dish towel and shake dry.
Stir together the salt, pepper, sugar, vinegar, dill, sour cream, and mustard, then add cucumbers and onions. Refrigerate for 1 hour or up to overnight before serving to let the flavors mingle.