Preheat the oven to 425°F. Place the onions in a small bowl, add enough cold water to cover and let sit 30 minutes; this draws out the strong flavor of the onion. Drain the onions well and put them back in the bowl, add the vinegar and toss to coat. Cover and refrigerate for at least 30 minutes and up to 3 hours. In a large bowl, sift the flour with the baking powder, salt, mustard and black pepper Cut in the shortening until the mixture has the consistency of small peas, then stir in the green onions and parsley. Lightly stir in enough milk with a fork to make a soft dough. Turn the dough out onto a lightly floured surface. Gently pat the dough out to a 3/4-inch thick-ness. Sprinkle the dough with a little flour. Cut into 15 rounds with a 1-1/2-inch biscuit cutter, pressing straight down into the dough without turning the cutter If you do not have a small biscuit or cookie cutter, then cut with a knife into small squares. Put the biscuits, sides slightly touching, on a baking sheet. (Note: The biscuits can be prepared to this point then covered tightly and refrigerated up to 4 hours before baking.) Bake until golden and fluffy, 18 to 20 minutes. Let the biscuits cool for about 10 minutes. Split the biscuits in half horizontally. Spread the biscuit bottoms with smoked salmon spread and top with a little of the pickled onion and a dill sprig. Arrange the biscuits on a serving platter, replace the top halves of the biscuits slightly askew and serve immediately.