1. Preheat oven to 400 degrees. On a lightly floured work surface, roll out puff pastry to a 12-inch square; transfer to a parchment-lined baking sheet. In a small bowl, stir together sour cream and mustard and season with salt and pepper. Spread sour cream mixture evenly over pastry. Fold over 1/2-inch border on all sides and press edges to seal. Refrigerate 10 minutes.

2. Meanwhile, halve leeks lengthwise; rinse thoroughly, pat dry, and slice 1/4-inch thick. In a large skillet, melt butter over medium-high. Add leeks, season with salt and pepper, and cook, stirring frequently, until soft, 3 minutes. Reduce heat to medium and cook, stirring frequently, until leeks are very soft, 5 minutes. Arrange leeks evenly over sour cream mixture on pastry. Top with tomatoes and season with salt and pepper. Bake until pastry is golden brown and crisp, 25 minutes. Let cool slightly. Top with goat cheese and herbs before cutting into squares. Serve warm or at room temperature.