For citrus-marinated cucumbers and onions: Combine water, vinegar, sugar and salt in a small saucepan and place over medium-high heat. Bring just to a boil and remove from heat. Add lemon slices to hot mixture. Allow to cool to room temperature. Put sliced cucumbers and onions in a small container and pour cooled marinade and lemon slices over them. Refrigerate at least 3 hours or overnight. Drain and remove lemon slices. For dill-mustard spread: Combine mustard, mayonnaise, sour cream and dill in a small container and set aside. For sandwich: Heat a large nonstick skillet over low heat. Spread ½ tablespoon butter on one side of each slice of bread. Put each slice of bread butter-face down in the skillet and top each with cheese slices. When cheese begins to melt, top one slice of each sandwich with 3 ounces smoked salmon. Top salmon with a layer of mustard-dill spread, then cover with citrus-marinated cucumbers and onions. Top with watercress leaves. When cheese has completely melted, use a spatula to put the sandwich halves together and remove from heat.
I'm a life-long cheese lover who enjoys being creative in the kitchen.