In a medium saucepan melt the butter until it's foaming then add the flour. Stir together until you have a smooth paste and keep cooking until it smells a bit biscuity - roughly 30 seconds.
Add the paprika & mustard powder then, in increments, add the milk, stirring vigorously each time and allowing the sauce to bubble up and thicken.
Turn the heat down low and add the cheddar and cooked pasta, stirring the cheese through to make a luscious sauce. Season to taste. Take the sauce off the heat and gently stir the mozzarella through it.
Pour into a narrow rectangular loaf tin - make sure it's lined first! Leave to set completely in the fridge.
Now, move on to the tomato sauce. In a medium saucepan fry the onion in the olive oil until translucent but not browned. Add the salt and chilli flakes. Add the tomato puree and stir it around to cook it out. Pour in passata and chopped tomatoes then season with black pepper and caster sugar. Put a lid on top and allow to bubble for 30 mins or so until thick. Leave to cool slightly off the heat.
Mix together all the ingredients for the meatball. On a lined baking tray, fashion 1/3 of the mince mix into a rectangle a little bit longer than the cheese ingot. Place the ingot on top and use the rest of the mince to encase it. Place in the oven and cook for 25 mins.
Remove from the oven and cover with the tomato sauce. Sprinkle with extra grated mozzarella and place back in the oven for a remaining 20 mins until the cheese is melted and bubbling.