For the tomato tart, preheat the oven to 210C/190C Fan/Gas 7.
Roll out the pastry on a lightly floured work surface to form a large square about 3mm thick.
Cut 4 x 17.5cm/7in circles from the pastry using an upturned bowl or cake tin. Transfer each to a baking tray.
Using a sharp knife, score a 1.5cm/½in border around the edge of each pastry circle. Brush each border with a little of the egg yolk.
Pile a quarter of the grated Gruyère in the centre of each pastry circle. Arrange the tomato slices in concentric circles on top of the cheese. Season, to taste, with salt and freshly ground black pepper, then scatter over the thyme leaves and drizzle with a little rapeseed oil.
Bake the tarts in the oven for 12-15 minutes, or until the pastry borders have risen and are golden-brown and the filling is bubbling.
Meanwhile, for the tomato salad, lay the tomatoes, red onion and lettuce on a roasting tray and drizzle over a little of the vegetable oil. Mix well to coat everything in the oil.
Place the roasting tray onto a heatproof surface and, using a chefs' blow torch, char until caramelised – this should take 4-5 minutes. Alternatively, preheat your grill to its highest setting and grill the vegetables for 4-5 minutes.
Shake the vinegar, mustard, egg yolk and the remaining vegetable oil in a clean, sealed jam jar until well combined, then season, to taste, with salt and freshly ground black pepper.
To serve, place a tart on each serving plate. Transfer the blow-torched vegetables to a large bowl and drizzle over enough of the dressing to coat the leaves. Mix the salad to coat it in the dressing, then serve.