Combine 1 c. apple cider, 2 Golden Delicious apples, peeled and coarsely grated, 2 shallots finely chopped, 2 tbsp apple cider vinegar, 1 tsp curry powder, and salt and pepper to taste in a saucepan and bring to a boil. Cook until the liquid evaporates, about 12 minutes; let cool slightly. Stir in 2 tbsp chopped cilantro. Meanwhile, pulse 1 ¼ lbs salmon skinless and cut into 1 inch chucks, in a food processor until finely ground. Transfer to a bowl; stir in 1/2 c. panko breadcrumbs, 2 tbsp mayo and 1 shallot finely chopped, 2 tbsp chopped cilantro, 1 tbsp apple cider vinegar and 2 tsp curry powder. Add salt and pepper. Form into four 1/2-inch-thick patties. Melt 2 tsp unsalted butter in a large nonstick skillet over medium-high heat. Add the patties and cook until browned and cooked through, 2 to 3 minutes per side. Butter the potato buns and toast and top the sandwiches with the salmon burgers, mayonnaise, the apple relish and more cilantro.