For the pastry, in a large bowl, rub together the flour and butter with your fingertips until the mixture resembles breadcrumbs. Season with a pinch of salt.
Add the beaten egg and bring the mixture together into a ball, then knead very lightly on a lightly floured work surface.
Wrap in greaseproof paper and cling film, then chill in the fridge for 10-15 minutes to firm up.
Meanwhile, for the filling, preheat the oven to 180C/160C Fan/Gas 4. Grease 6 small, loose-bottomed tart tins, each with a diameter of 9cm/3½in diameter and a height of 4cm/1½in.
Roll the pastry out onto a lightly floured surface to a thickness of 3mm. Cut 6 discs, each slightly larger in diameter than the tart cases, from the pastry (you will need to re-roll the pastry off-cuts for the last two discs).
Line each tin with a pastry disc, pressing gently into the base and sides, then trim the sides so that the pastry cases are flush to the edge of the tins.
Dry fry the bacon in a frying pan over a medium heat for 3-4 minutes, or until golden-brown and just crisp. Drain on kitchen paper, then roughly chop and set aside.
Whisk together the eggs and cream in a jug, then season, to taste, with salt and freshly ground black pepper.
Sprinkle half the cheese equally among the tart cases, followed by half of the chopped bacon.
Divide all of the diced courgette among the pastry cases, then cover with the remaining cheese and bacon. Garnish with the thyme leaves.
Pour the egg and cream mixture into the tart cases, then bake in the oven for 15-20 minutes, or until the filling has just set and is golden-brown on top. Remove from the oven and set aside to cool slightly before serving.
Meanwhile, for the salad, shake the mustard, vinegar and rapeseed oil in a small jar until well combined.
Fill a resealable plastic bag with the salad leaves, add the dressing and shake the bag to coat the leaves in the dressing. Alternatively, pour the dressing over the salad leaves in a bowl and mix well.
Serve the quiches warm with the salad alongside.