Preheat the oven to 210°C.
Roll out the pastry to 3mm thick on a floured surface.
Cut four 17.5cm circles and place them on a baking tray.
Score a 1.5cm border on each circle and brush with egg yolk.
Top pastry centers with Gruyère, tomato slices, salt, pepper, thyme, and rapeseed oil.
Bake for 12 to 15 minutes until golden and bubbling.
Coat salad tomatoes, red onion, and lettuce with vegetable oil on a roasting tray.
Char the vegetables with a blow torch for 4 to 5 minutes until caramelised.
Combine vinegar, mustard, egg yolk, and remaining oil in a jar to make dressing.
Dress the charred salad and serve alongside the tarts.