Preheat your oven to 425°F (220°C). Place the sliced tomatoes on a large baking tray, then drizzle with olive oil and season with salt and pepper. Roast for 30 minutes until the tomatoes are soft and caramelized around the edges. Remove from the oven and allow to cool while preparing the tart's crust.

Turn the oven down to 400°F (200°C). Line a baking tray with parchment paper. Place the puff pastry sheet on the lined tray. Using a small thin knife score the edges of the pastry, creating a 1/2 inch border around all 4 edges of the tart. This will create the crust of the tart when baking. Bake the crust as is for 8-10 minutes until lightly browned and slightly puffed.

Spread the pesto all across the base of the pre-baked tart. Layer on the roasted tomatoes and mozzarella slices. Once the whole tart has been topped brush the outer edged of the tart with egg wash. Place the assembled tart back in the oven and allow to bake for 15-20 minutes or until the pastry is golden and crisp and the cheese has melted. Allow to cool slightly then garnish with fresh basil. Cut into 12 squares and serve. Cover and store the tart in the fridge for up to 3 days. To reheat, toast in the oven or warm in the microwave.