• Preheat your oven to 180c (350f)

For the panko crumbs

• Melt the butter over medium heat for 3 - 4 minutes, swirling the pan until it browns then take off the heat and mix into the panko crumbs straight away.
• Add the rosemary leaves in and then set the crumbs aside.

For the sauce

• Heat the olive oil and butter over a medium-high heat and fry the bacon until crisp and golden, about 10 minutes.
• Remove with a slotted spoon and set aside.
• Toss the leeks in the bacon fat and then cook, stirring often for five minutes. Lower the heat to low and continue cooking for a further 5 - 6 minutes until really soft. Don't let them catch or burn so keep an eye on them.
• Set aside with the bacon and make the cheese sauce.
• In a large saucepan heat the butter over a medium heat and when sizzling, add the flour.
• Whisk vigorously for a minute.
• Lower the heat slightly and add the milk in a slow stream, whisking the whole time making sure you get all the flour mixture from the bottom and sides of the pan.
• Cook for a few minutes until it thickens slightly and coats the back of a wooden spoon.
• When ready it should almost be at a slight simmer but not quite.
• Take off the heat and add the cheese in three lots, whisking well and letting it melt in between each addition.
• Add the cook pasta, bacon, leeks and pepper in and mix well.
• Season with a couple of big pinches of salt and add more pepper if you wish.
• Pour into a 9 x 9 inch pan (see notes) and top with the panko crumbs in an even layer.
• Bake in the oven for 25 minutes until nice and golden. You don't want to bake it any longer than that as you don't want to overcook the pasta and dry out the sauce
• Leave it to sit for 10 minutes before digging in!