Preheat oven to 220ºC (200ºC for fan assisted ovens)/Gas M7
Unroll pastry sheet onto a lined baking tin and spread the pesto over top leaving a border all round so the pastry can puff up. Brush border with beaten egg.
Top with overlapping tomato and onion slices, drizzle vegetables with a very little oil and scatter with a few thyme leaves. Bake for 20 minutes until borders have risen and are golden. Scatter with remaining thyme and serve.