On a lightly floured surface, roll out the puff pastry to thin it out a bit. Transfer to a parchment-lined baking sheet.
In a small bowl, mix sour cream and mustard. Season with salt and pepper. Spread sour cream mixture evenly over pastry; fold over a ½ inch border around the edges of the pastry and press to seal. Place baking sheet in the refrigerator for about 10 minutes.
Chop leeks, place in a fine sieve. Rinse well and then pat dry thoroughly. In a medium skillet, melt butter over medium heat. Add leeks and season with salt and pepper. Saute, stirring frequently, until leeks are very soft (about 6 minutes).
While leeks are cooking, set tomato slices on paper towels to allow the towels to soak up some of the juices from the tomatoes.
Arrange cooked leeks over the sour cream mixture on the chilled pastry. Top with sliced tomatoes. Season with salt and pepper.
Bake until pastry is golden brown and crisp, about 25 minutes. Let cool slightly, then top with goat cheese and basil leaves.
Serve warm or at room temperature.
Notes
Sub halved pear tomatoes for the small, sliced tomatoes if you'd like.