Heat the sugar and 50ml/2fl oz water in a saucepan until the sugar melts and the caramel is golden-brown. Remove the pan from the heat and stir in the butter until smooth. Divide the caramel between four x 13cm/5in ovenproof dishes.
Sprinkle the sea salt and thyme over the caramel and lay the chicory heads on top. Press the pastry circles onto the chicory and trim the edges to fit. Place the dishes into the oven and cook for 20 minutes, or until the pastry is crisp and golden-brown.
For the vinaigrette, blend the cheese, garlic, mustard and white wine vinegar in a food processor. With the motor on a slow setting, gradually add the oil and seasoning and blend until smooth and well combined.
To serve, invert the tarts onto serving plates and drizzle over the vinaigrette. Sprinkle over the walnuts and mustard cress.