Pre heat the oven at 395 F = 200 C.
Mix sugar, cinnamon and walnuts together.
Lightly roll out the crescent dough on a piece of baking paper larger than the final size of the dough. Lightly flour the dough to make rolling easier.
Optional - if you want a "prettier" looking tart, cut the dough in a round shape.
Grab the baking paper from the sides and transfer dough to the baking dish, leaving a border.
Spread plums inside the dough.
Sprinkle the sugar mixture.
Fold in the edges of dough to cover outer rim of plums, pleating dough as necessary.
Bake in the middle of the oven for about 20 minutes or until the tart gets a golden color.
Remove from the oven and leave to cool in the baking dish.
Cut into squares or slices and serve simple, with whipped cream or sour cream.