Make the vinaigrette: in a small bowl, whisk the lemon juice, shallots, dill (and mustard, if using). Slowly add the olive oil, whisking constantly. Season with salt and pepper.
Just before the party, arrange the endive spears on a platter and lay a strip of smoked salmon on each one. Drizzle with vinaigrette and garnish with dill leaves and lemon wedges.
MAKE AHEAD: The vinaigrette keeps in the refrigerator for a day. You can also prepare a platter of endive spears with smoked salmon (without vinaigrette) several hours before the party; wrap with plastic wrap and keep chilled in the refrigerator. Drizzle with the vinaigrette just before serving.