Start by mixing the first three ingredients until well blended, then set aside.
Next, trim the ends off the baguette, cut in half crosswise, then cut in half horizontally.
On the bottom half of each section, spread the Dijon mustard to taste, careful not to spread too thickly.
Then spread the compound butter over the Dijon mustard, equally between each section.
Next, stack half of the watercress and half of the smoked salmon over the butter.
Place the top of the baguette over each section and cut each section into 4 equal parts.