Preheat the oven to 200°C/180°C fan. Bring a large pot of salted water to the boil. Once boiling, add the pasta and cook for 8-10 min until 'al dente' or cooked to your liking. Drain and reserve a cup of pasta water.

Meanwhile, peel and finely dice the onion. Roughly chop the spinach. Drain the tuna. 

Heat a large pan over a medium heat with the butter. Fry the onion with a pinch of salt for 5-6 min until soft. 

Once soft, add the garlic powder and flour and cook, stirring, for 1 min until a sandy paste forms. Once a paste has formed, whisk in the milk and cook for 3-5 min further or until the sauce has thickened. Add the measured water (use the pasta water), stock cube, oregano, mozzarella, spinach and Worcestershire sauce. Cook for 1 final min and season with pepper.

Add the drained pasta to a large baking dish with the drained tuna and pour the sauce over the top. Give everything a good mix up. Top with the grated cheddar cheese and bake for 15 min until golden brown.