Trim pastry sheet edges; cut sheet into fifteen 3x2-inch rectangles. Place on baking sheet. Combine onion and snipped dill. Press 2 teaspoons onion mixture in the center of each rectangle.
Stir oil and pepper into remaining onion mixture. Cut potatoes into 1/8-inch slices; overlap four or five slices in center of each rectangle. Drizzle with onion-olive oil mixture. Cover; freeze just until firm. Transfer to freezer container; seal, label and freeze up to 1 month.
Uncover; bake in a 400 degreee F oven for 20 to 25 minutes or until edges of pastry are puffed and brown and potatoes are tender. Top tarts with sour cream and salmon. Serve warm; garnish with dill sprigs. Makes 15 side-dish servings.