Preheat the oven to 180 degrees celcius
Roll out the puff pastry and fold in the edges a bit so the filling won't fall off.
Melt the butter and add the thyme. Lightly brush the pastry with the butter. Divide the cheese on top.
Clean the onions and cut in half rings. Put on the tart, you can choose to do it in a pattern.
Bake a total of 40 minutes but check regularly if the pastry isn't getting too brown. Cover the pastry with foil if necessary. After covering with foil you cab put the broiler on so the onion will caremalize quicker.