2 bunches watercress, stems removed
1 pound thinly sliced smoked salmon
2 teaspoons prepared horseradish
2 teaspoons coarse-grain mustard
1/2 cup sour cream
1/4 cup fresh lemon juice
1/2 teaspoon kosher salt
1/4 teaspoon black pepper
1/2 hothouse (seedless) cucumber or 1 regular cucumber, quartered lengthwise and thickly sliced
2 tablespoons finely chopped fresh dill
1 lemon, cut into wedges (optional)
bagel chips or 1 thinly sliced baguette