Preheat oven to 220ºC (200ºC fan assisted)/425ºF/Gas 7.
Roll out pastry to a rectangle approx 24 x 34 cm/9.5” x 13.5”. Using a 5 cm/1.5” cutter, cut out 24 discs. Place on a baking sheet.
Cut the tomatoes in half through their ‘equator’, cut each onion into half and then each half into 6 wedges. Brush cut surface of tomatoes and onion very lightly with oil.
Brush each pastry disc with beaten egg, then place a halved tomato and onion wedge in centre of each. Bake for 10-12 minutes until pastry is risen and golden. Serve warm, garnished with a small basil leaf.