Place cucumbers into a large bowl with a lid, mix in 1 tablespoon salt, and cover. Refrigerate to let salt draw out excess moisture, 2 to 3 hours. Drain liquid and squeeze as much moisture as possible out of cucumbers; transfer to a large salad bowl.
Mix cider vinegar, evaporated milk, vegetable oil, 2 teaspoons salt, and black pepper in a jar, shaking it to thoroughly combine. Pour dressing over cucumbers and stir in onion. Refrigerate at least 4 hours (overnight for better flavor).
