Place the sliced cucumbers in a colander set over a bowl or in the sink. Sprinkle with ½ teaspoon of the kosher salt and toss gently. Let them sit for 15 minutes to draw out excess moisture, then pat them very dry with paper towels or a clean kitchen towel.
In a large mixing bowl, combine the softened cream cheese, sour cream, lemon juice, Dijon mustard, remaining ½ teaspoon salt, and black pepper. Use a spatula or wooden spoon to mix until smooth and well combined.
Add the dried cucumber slices, flaked smoked salmon, chopped dill, and red onion (if using) to the bowl with the cream cheese mixture.
Gently fold everything together until the cucumbers and salmon are evenly coated with the creamy dressing. Be careful not to overmix—you want to keep some texture in the salmon.
Transfer the salad to a serving dish, cover, and refrigerate for at least 30 minutes to allow the flavors to blend. Garnish with extra fresh dill just before serving.