Preheat your oven to 350f (180c).
Make the topping first. Mix all topping ingredients together, add a good pinch of salt and set aside.
Cook the pasta, undercooking by 1 minute from package instructions. Drain and toss with 1 tablespoon of butter (helps it stop cooking and sticking).
Make the 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.
Lower the heat right down and add the cheese in three lots, whisking well and letting it melt in between each addition.
If there are still some small chunks not melted in, it's fine!
Add the rosemary, black pepper and lemon juice and mix in.
Add the pasta, mix and season to taste with salt.
Pour into your baking dish (I used a 30cm x 21cm rectangular dish) and top with the panko crumb mixture.
Bake in the oven for 20 - 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 5 - 10 mins before serving.