1. Preheat the grill to medium-high. Heat a little olive oil and 25g of the butter in a frying pan, add the leeks and cook gently for 8-10 minutes until the white part of the leeks are softened. Remove from the heat and set aside.
2. In a separate pan, melt the butter and whisk in the flour. Cook for 2-3 minutes until the mixture starts to turn a light brown colour. Gradually whisk in the milk; don’t add too much at once or the sauce will turn lumpy. Once all the milk has been incorporated bring the sauce to a simmer and cook gently for 2 minutes, whisking constantly. Remove from the heat, whisk in the cheese and season with salt and pepper.
3. Rub the inside of the square Pyrex dish with the garlic then arrange the leeks in an even layer in the bottom. Pour over the sauce and sprinkle over the Parmesan cheese and breadcrumbs. Place under the grill for 5-6 minutes until golden brown und bubbling. Leave to stand for a few minutes before serving.