Heat the oil in a frying pan over a medium-low heat and fry the garlic and onion gently until they are just starting to colour. Add the tomatoes, olives and thyme and simmer, mashing well with a wooden spoon.
Cook for about 10 minutes to allow the sauce to thicken. Stir in the tuna and simmer again for about a minute. Season and leave to cool.
Preheat the oven to 200°C and oil a baking sheet.
Roll the pastry into a sausage shape and slice into 15 pieces. Roll out each piece of pastry on a well-floured board into a thin circle, the size of a saucer. Fill with a heaped teaspoon of the filling, wet the edges and fold one half over the other to enclose the filling, sealing with a fork. Continue until you have 15 or so little pasties.
Transfer to the baking sheet. Bake for about 10-15 minutes, until puffed and brown. Serve warm garnished with sliced tomato.