- Toss the tomatoes with the red onion, garlic, olive oil and vinegar and let it sit, covered, for 30 minutes (or up to 3 hours in the fridge). Strain the tomato mixture, saving the juices. Bring the reserved juices up to a simmer and reduce to just 1 to 2 tbsp, then pour over the tomatoes and toss (this concentrates the flavours). Stir in the basil and season to taste.
- Preheat the oven to 375°F and line a baking tray with parchment paper. Unfold the puff pastry on a lightly floured surface. Cut the puff pastry into 4 squares and place them on the prepared baking tray. Spoon tomato filling into the centre of each square and fold up the edges, keeping the square shape. Brush the edges of the pastry with egg mixture.
- Bake for 30 to 35 minutes, until pastry is a rich golden brown colour. Crumble goat cheese over tarts and return to oven for 5 minutes. Serve the tarts warm or at room temperature.