1. First make the coulis. Heat your oven to 220°C. Put the tomatoes, cut side up, in a roasting tin. Add garlic and trickle over the balsamic vinegar. Drizzle generously with olive oil. Sprinkle with the sugar and season lightly with salt and coarse pepper. Roast for 20-30 minutes or until very soft and juicy, and slightly coloured on top (out-of-season tomatoes may take longer). Make sure that they don’t burn. Allow to cool. Peel the garlic and put the cloves, along with the tomatoes and any juices from the roasting tin, into a blender. Add the basil and a bit more olive oil. Blitz thoroughly, then pass through a sieve into a bowl to make a smooth sauce. Check the seasoning, cover and chill until needed.
2. Meanwhile, for the topping, heat 1 tbsp olive oil in a frying pan and gently fry the onion until softened. Add a sprinkling of sugar and cook for another couple of minutes until the sugar has dissolved. Set aside.
3. Plunge the courgette slices into a saucepan of boiling water, blanch for a minute or two, then drain, pat dry and leave to cool.
4. Line a baking tray with baking parchment or silicone paper.
5. Roll out your puff pastry on a lightly floured surface to a large circle, about 2mm thick. Using a large plate as a guide, cut out a 26cm round. Prick all over with a fork and use the tip of the knife to lightly mark a 2cm boarder all around the edge. Place on the prepared baking tray. Chill for 30 minutes.
6. Heat your oven to 200°C. Spoon the red onion over the pastry within the boarder and then arrange the courgette slices in layers over the top, over lapping them slightly. Sprinkle with olive oil and season with salt and pepper. Brush the pastry boarder with beaten egg.
7. Bake for 20-25 minutes until the pastry is puffed and golden brown. Allow to cool. Before serving, brush with a little more olive oil and squeeze over some lemon juice. Finish with a fine grating of Parmesan and a few twists of the pepper mill. Serve with a spoonful of the tomato coulis on the side and a light summer salad.