Add the sliced cucumbers to colander over a bowl and mix in the 1 tablespoon of salt to help release as much liquid as possible. Let it stand for 20-30 minutes or up to 6 hours. If salting for longer than 30 minutes, place it in the refrigerator.
Mix the mayonnaise, vinegar, sugar, and dill, and set aside.
Gently press down the cucumbers with a spatula and then transfer them to a large clean bowl and toss with the dressing, onions, salt, and pepper. Serve.
Make-Ahead: You can make this up to 6 hours ahead of time, however, be sure to keep the dressing separate from the cucumbers before serving.How to Store: Cover and keep the cucumber salad in the refrigerator for up to 3 days.If you are serving immediately, you most likely will not need to salt the cucumbers ahead of time.