For the rosti: grate 1 large russet potato into a bowl. Sprinkle a good pinch of salt, at least 1 tsp over the potato and mix. Let sit for 5 minutes and in batches, squeeze the water out of the mixture in a kitchen towel (to the best of your ability).
Add a couple tsp of vegetable oil to a cast iron skillet on medium low. The idea is to make thin, crispy "pancakes" that are halfway to a potato chip. Each rosti probably contains 2 tbsp of potato. Spoon some out, and flatten in the pan, spread out as thin and even as you can. Fry slow and steady. You don't want the outside going too dark before the middle cooks. Drain on paper towel once they are a deep golden brown and crispy.
Mix all ingredients for cream cheese mixture (everything except the potato/oil and smoked salmon for the top) and chill.
Smoked salmon: The key here is a nice sharp knife and patience. Slicing the salmon as thin as possible makes eating this hors d'oeuvre so much easier. We buy entire sides from T&J but you can purchase in smaller quantities. You can play it according to preference here.
To build the rosti, scoop a heaping tbsp of the cream cheese mixture on top of the rosti and layer the amount of sliced smoked salmon that feels right to you. Top with pickled onions to taste (Christie loves extra pickled onions!).
Instructions for "Quick-Pickled Onions"
Pack the onions into a 1-pint mason jar or similar heat-safe vessel. Place the jar in the sink, to catch any splashes of hot vinegar later.
In a small saucepan, combine the water, both vinegars, maple syrup, salt, and pepper flakes. Bring the mixture to a gentle simmer over medium heat, then carefully pour the mixture into the jar over the onions.
Use a butter knife or spoon to press the onions down into the vinegar and pop any air bubbles in the jar. Let the pickled onions cool to room temperature (about 20 to 30 minutes), at which point they should be sufficiently pickled for serving.
Cover and refrigerate leftover pickled onions for later. Quick-pickled onions are best consumed within five days, but they keep for 2 to 3 weeks in the refrigerator.
