Step 1

Preheat the oven to 190C/374F.

Step 2

Heat the butter in a saucepan. Add the chopped onion and cook over a low heat until the onion is soft, about 20 minutes should do the trick.

Step 3

Add the flour to the onion and stir well, until you have a smooth paste.Switch from using a wooden spoon to a whisk then start adding the milk slowly, whisking continuously.Once all the milk has been added leave the sauce to simmer for a minute or two then whisk in 1 tbsp of wholegrain mustard, 2 cloves of minced garlic and 1 tbsp of thyme leaves. Simmer again for a few minutes, the sauce should now be silky smooth.Whisk in 2-3 tbsp of water from the cooking pasta.

Step 4

Turn off the heat then add 70g of grated cheddar and 30g of finely grated parmesan. Stir well and season with salt and black pepper. Set aside until you're ready to use it.

Step 5

Cook the pasta according to the packet instructions minus 5 minutes.About 5 minutes before it is due to be drained add the broccoli florets and cook them both together for about 5 minutes.Drain the pasta and broccoli then tip it all into a large casserole dish.

Step 6

Pour the sauce over the pasta and broccoli and gently mix it all together.In a small bowl mix together the panko breadcrumbs and the remaining cheddar and parmesan.Sprinkle the cheese and breadcrumb mixture over the pasta then cover the dish loosely with kitchen foil. Place the dish in the oven and cook for 20 minutes. Remove the foil then transfer the dish to under the grill and cook for about 10 minutes or until the topping is golden brown and crispy and the cheese has melted.Leave it to cool slightly then serve. If you want you can sprinkle some finely chopped parsley over the finished dish.