How to cook
Use a knife to make two narrow incisions on the top of the cheese. They should not be deep, but enough so they pierce its crust.
Put the cheese to cook in a pan for about 5 minutes. Just before the cheese is ready, cut the bread in half and grease it with a little butter. Toast for about a minute with the cheese, toasting only on the inside.
Meanwhile, melt the butter in another pan. Slice the onion and add it to the melted butter, sprinkle with half the sugar.
Wait until the onion turns golden, turn the slices over and sprinkle with the rest of the sugar. Cook for another 1-2 minutes, remove from heat. Slice the tomatoes - you need one or two.