Preheat oven to 425°F. Cut out a 10-inch circle of parchment paper or foil and coat one side with cooking spray.
Place potatoes in a large saucepan, cover with cold water and add salt to taste. Cover and bring to a boil. When the water reaches a full boil, drain the potatoes and refresh under cold water. (They will not be fully cooked.) Pat dry.
Shred the potatoes (a food processor fitted with a shredding disc works well) and place in a large bowl. Add mustard, salt and pepper; toss to combine. Gently mix in smoked trout and scallions.
Heat oil and butter in a medium cast-iron or other ovenproof skillet over medium heat. Use a pastry brush to spread the fat evenly over the bottom and about 1 inch up the sides of the skillet. Add the potato mixture and press firmly into a large pancake. Place the paper or foil circle, sprayed-side down, directly on top of the potatoes. Transfer the pan to the oven and bake until the potatoes are golden on the bottom, about 30 minutes.
Remove the paper or foil circle and use a thin metal spatula to loosen bottom and sides of the rosti. Invert a serving plate over the pan. Grasping the pan and plate (protect hands with oven mitts), invert and unmold the rosti. Cut into wedges and serve with Zesty Horseradish Cream.