Mix the flour and salt in a bowl. Melt the butter in a pan. Remove the pan from the heat, pour in the water, stir, add to the flour in one go, stir with a wooden spoon to form a smooth, firm dough; do not knead. Flatten the dough, cover and leave to chill for approx. 1 hr.
Heat the butter in a frying pan, add the onions and stir fry for approx. 5 mins. Pour in the balsamic and water, add the thyme, reduce the heat. Simmer the onions for approx. 10 mins., season.
Divide the dough into 16 portions. On a lightly floured surface, roll out the pieces of dough into circles (approx. 10 cm in diameter). Place approx. 1 tsp of cream cheese in the centre of each galette, top with approx. 1 1/2 tbsp of filling. Fold the edges inwards, making sure the filling can still be seen.
Approx. 25 mins. in the lower half of an oven preheated to 220°C. Remove from the oven, transfer to a cooling rack and leave to cool.