If using frozen pastry, allow it to defrost.
Preheat the oven to 220C/430F/Gas Mark 7.
Roll the puff pastry outto a thickness of 3mm. Cut into four round circles, each the size of a side plate.
Using a sharp knife, cut a small edge from around the pastry and lay this piece around the outer edge of the tart, to make a double-layered frame.
Break the egg into a bowl and whisk with a fork. Brush the pastry circles with the egg wash around the edges.
Using a fork, pierce the pastry bases a few times and then place in the oven to bake for 15-20 minutes, until the pastry bases are golden-brown and cooked.
Meanwhile, melt the butter in a large pan. Add the onions, garlic and thyme and cook gently for 15 minutes, until deep brown and caramelised.
Season to taste with salt and freshly ground black pepper.
Remove the cooked pastry bases from the oven and spread with the crème fraîche.
Top with the caramelised onions, drizzle with olive oil and return to the oven for a few minutes.
Meanwhile, in a large clean bowl, mix the balsamic vinegar with the olive oil and some salt and freshly ground black pepper. Add the salad leaves and toss well to coat.
To serve, remove the tarts from the oven and place on a large serving plate. Drizzle with a little olive oil and serve with a small pile of the dressed salad in the middle of each tart.