Heat about a tablespoon of oil in a frying pan, and add the sliced leek and garlic. Cook over a medium heat for a few minutes, until soft and fragrant. Set aside.

Melt the butter in a small saucepan, and add the flour. Stirring constantly, cook over a low heat, for a minute or so to cook the flour. Add the milk a little at a time, whisking until smooth each time before adding more.

When the béchamel sauce is smooth, but fairly thick, add the sautéed leeks and crumbled feta, along with plenty of black pepper. Set aside to cool for about 15 minutes.

Spread the breadcrumbs out on a plate. Take about one heaped tablespoon of the béchamel sauce, and shape it into a log (or your desired shape). Roll it in the breadcrumbs, and repeat with the remaining sauce.

Heat some more oil in a frying pan (you can use the same pan as you did before) - just enough to comfortably cover the bottom of the pan. Add the croquettes, making sure that you're not overcrowding the pan (you might need to cook them in two batches). Cook over a medium-high heat for around 5 minutes, rolling them over occasionally, until golden brown all over. If cooking the croquettes in more than one batch, remove any burnt breadcrumbs from the pan before adding the next batch.