The tarts I made for our starter on Christmas Day were just squares of puff pastry, with a cut 1cm from the edge, all the way round, to make the raised edges when they cooked. I caramelised a load of sliced onion, half a dozen smallish ones, with about an ounce of butter, put the lid on a frying pan until they softened then added a little sugar, just 1tsp, until it went brown. They went into the middle of the pastry squares then slices of the cheese.
I did make a couple of them with brie instead as B and H don't like goats' cheese. A sprinkle of thyme on the top and into a hot oven for 10 minutes! They were super-delish but with hindsight I would have cooked them fresh on the day and eaten them straight from the oven. I cooked them in advance on Christmas Eve and reheated them, which was fine for the taste but the pastry was a little too crumbly with being reheated! Still, they were a hit I got the recipe from the BBC Good Food website