Preheat oven to 180˚C fan / 200˚C /360˚F convection / 390˚F.
In a large saucepan bring 1 litre (4.25 cups) water to the boil, break the cauliflower into large florets then place in the boiling water and cook for 5-10 minutes, until the cauliflower is just tender.
Drain the cauliflower in a sieve over a bowl, keeping the water to use in the sauce.
Meanwhile, cook the pasta in boiling water according to packet instructions – around 10 minutes. Drain.
In a medium saucepan over a low heat add the butter. Once it’s melted add the flour and stir until well combined. Cook for a couple of minutes, until it starts to bubble, then gradually add the 400ml (1 ¾ c) milk and 400ml (1 ¾ c) cauliflower stock, whisking with a balloon whisk between each addition. Bring to a simmer and cook for a couple of minutes until it starts to thicken.
Bring to a simmer and cook for a couple of minutes until it starts to thicken. Take off the heat and stir in the cheese, dash of Worcester sauce and then the tuna. Season with black pepper.
Tip the cauliflower and pasta into an oven proof dish, then pour over the sauce. Mix to thoroughly coat the past and cauliflower in sauce, then sprinkle over the remaining 25g / 1.5 tablespoons of grated cheese.
Place in the preheated oven and cook for 30-35 mins until golden and bubbling. Stand for a couple of minutes before serving.
Notes
* in the UK tins of tuna weigh 145g rather than 185g, use one tin.