Lightly grease a large baking sheet and sprinkle with water. Halve about 150 g (5 oz) of the tomatoes. Heat the oil in a frying pan, add the onion and fry for about 3 minutes until softened. Remove the pan from the heat, add the garlic and sun-dried tomato paste, then stir in all the tomatoes, turning until they are lightly coated in the sauce. Chill and set aside until required.
Roll out the pastry on a lightly floured surface and cut out four 12 cm (5 inch) rounds using a cutter or small bowl as a guide. Transfer to the prepared baking sheet and make a shallow cut 1 cm (½ inch) in from the edge of each round using the tip of a sharp knife, to form a rim. Brush the rims with beaten egg. Cover, chill and set aside until required.
Lightly mix together the crème fraîche, pesto and salt and pepper in a bowl so that the crème fraîche is streaked with the pesto. Cover, chill and set aside until required.
When ready to serve, pile the tomato mixture on to the centres of the pastries, making sure the mixture stays within the rims. Bake the tartlets in a preheated oven, 220°C (425°F), Gas Mark 7, for about 15 minutes until the pastry is risen and golden. Transfer the tartlets to serving plates and spoon the crème fraîche and pesto mixture into a small serving dish. Serve scattered with basil leaves.