Pour oil into a 10- to 12-inch frying pan over medium-high heat; when hot, add leeks and cook, stirring occasionally, until very limp, about 10 minutes. Let cool to room temperature, about 15 minutes.
Meanwhile, peel potatoes and slice crosswise very thinly (1/16 in.). Lay puff pastry dough flat on a lightly floured board. Trim off corners to form a circle and, with a lightly floured rolling pin, roll out slightly to make an 11-inch round. Transfer to a 12- by 15-inch baking sheet.
Spread leeks in the center of dough, leaving a 2-inch border around the edge; sprinkle lightly with salt and pepper. Arrange potato slices in slightly overlapping layers over leeks, sprinkling each layer lightly with salt and pepper. Fold in edges of dough over potatoes. Drizzle cream over potatoes.
Bake in a 375° regular or convection oven, rotating baking sheet midway through baking time, until crust is brown and potatoes are tender when pierced, about 35 minutes (if crust is brown before potatoes are tender, cover tart loosely with foil).
Top tart with capers and smoked salmon slices while warm. Cut into edges and serve warm.