Heat olive oil in a heavy based pot. Add onions, salt & pepper; cook over a medium heat for 20 minutes until soft, stirring occasionally.
Add balsamic vinegar & sugar, remove the lid; cook a further 10 minutes, stirring occasionally. Set aside to cool.
Put potatoes in a medium pot, cover with water, season with salt & bring to a boil. Reduce heat to a simmer, cook potatoes 15-20 minutes until just able to be pierced with a knife. Drain & set aside to cool.
Roll out pastry to fit a 10-inch round removable base tart tin. Pop in the freezer to chill.
Preheat oven to 350 F.
Layer the onion, potato and chicken into the pastry shell. Dot with Boursin cheese and thyme. If you happen to be using left-over roast chicken & have a bit of stuffing spare, pop that in too! Whisk eggs and cream and pour over filling.
Bake for 35-40 minutes until the filling is set & pastry is golden.