Mary Berry isn’t afraid to switch up a classic recipe ... cooking off her leeks and forming a roux, she seasons the sauce and adds fish to it. Then, she spoons the pie base into a casserole ...

For the fish filling, melt the butter in a saucepan, add the leeks and stir over the heat. Cover with a lid and simmer gently for 10 minutes, or until soft. Measure the flour into a small bowl.

Add the flour to the leeks and stir well ... You can make the mash for this fish pie recipe in advance (mash freezes really well, it can try out a little but that's easily solved by stirring ...