Unroll the pastry and lay out on a baking sheet. If you don't have a baking sheet large enough for it to fit on, cut it in half and make two tarts.
Using a small sharp knife, score about an inch in from the side of the pastry. You don't want to cut right through it, but cut through about 75% of the pastry's layers.
Next use a fork to gently stab the pastry all over within the score line and then spread the mustard out again within the scored area.
Slice the tomatoes and cheese to about half an inch thick and cover the centre of the tart.
Sprinkle a little dried oregano and black pepper over the top and bake for around 20 minutes, or until the edges have puffed up and turned golden and the goats cheese has melted.