Place all pastry ingredients in a processor and pulse until it just comes together. (If using a Magimix CE, put all ingredients in steel bowl and run pastry program.)
Turn onto floured surface, pat lightly into a disc, wrap in plastic wrap and refrigerate for 30 minutes.
Preheat oven to 180C
Grease rectangular tart pan with removable base.
Roll chilled pastry to fit base and sides of tart pan.
Cover with a sheet of baking paper, weigh down with baking weights and bake for 15 minutes.
Remove paper and weights and bake for a further 5-10 minutes, until base is dried.
Allow to cool for 15 minutes.
While pastry is chilling and baking, heat olive oil over moderate heat, add onion slices and a good pinch of brown sugar and cook slowly until soft and caramelised.
Set aside to cool.
Spread base of cooled tart case with a thin layer of mustard, sprinkle with grated cheese and caramelised onions, then distribute cherry tomatoes on top, evenly.
Sprinkle thyme leaves.
Bake 180C for 30-40 minutes.
Tart is ready when the cheese is bubbling and the tomatoes are puckering.
Stand to cool for 10 minutes before serving.
