Pre heat oven to 200c.
Line a baking tray with baking/parchment paper, and place frozen pastry sheet on paper to begin to defrost.
Cut tomatoes in half lengthways - cut any larger ones into quarters lengthways.
Sit tomatoes on kitchen towel/absorbent paper to drain slightly - give them a gentle mix to help the process along.
Take a small sharp knife and score a square border about 1 1/2 cm or 1/2 inch from the edges of the pastry, being careful not to cut all the way through.
Take a fork and "dock" the pastry - a fancy name for making lots of little holes in it - right up to (but not over) the scored edge (these holes can go all the way through the pastry) - see my photos at the the end of the recipe for an example.
Evenly spread the mustard over the pastry, leaving the border.
Sprinkle the panko crumbs evenly over the mustard, again leaving the border.
Place tomatoes, cut side up, in an even layer of the pastry, again - you guessed it! - leaving the border.
Tuck prosciutto shreds in between tomatoes at random intervals.
Bake for 25 mins or until pastry is puffed and golden.
Top with crumbled feta and basil leaves, salt & pepper to taste then serve immediately.