The problem with tarts is that they are really easy to make. And the word can be interpreted so many ways. This recipe is loosely inspired by whatever Jamie Oliver show was on the other day. I think it was about strawberries, and I think he called this a free-form tart.
Heat the oven to 200 Degrees Celsius. Take half a sheet of puff pastry, and cut it into two squares. Quarter some fresh black plums and mix with a couple tablespoons of brown sugar. Sprinkle the pastry with more brown sugar, then pile half the plums in the middle of each square. Fold the pastry up around the edges, to make a loose parcel. Use some water or egg to make the pastry stick if it won't stay put. Bake in the oven for 15 minutes, or until the pastry is puffed and brown.