First of all, take a knife and chop the onion and sauté on the cutting board. Then, put the chopped onion and sauté into a saucepan. After that, add the mushrooms and the drained sweetcorn into the saucepan and cook them for around 2 to 3 minutes on low heat stove.
On the other hand, take a bowl and put cornflour into it. Now, add the extra milk and mix it well with the cornflour. Then, add the mixture to the saucepan containing the mushrooms, sweetcorn, onion and sauté. After heating it, add the cornflour paste into it.
Meanwhile, you have to grind the bread into breadcrumbs which you will add to the saucepan.
You have to roll out the thawed pastry into two rectangles (or you can roll it in any shape you like). Then, fill each of the thawed pastry with the mixture. You have to fold it in half. After folding, just press the open edges down.
Now, cook the pasties in the oven at gas mark 5 (190 degrees Centigrade or 375 degrees Fahrenheit) for around 20 to 30 minutes.
Your sweetcorn and mushroom puffs are ready to serve now. You can serve them right after cooking or you can wait for some time.