To make the white sauce, put the butter in a large saucepan and place over a low heat. Once melted, add the flour and stir in until it forms a paste, then pour in the milk and stir well. At this point, keep stirring until it thickens into a sauce, around 3-5 mins. Lastly, add the cheese and mustard, if using, then stir. Season to taste, then set aside.

Pour the garlic-infused oil into a large lidded pot over a medium heat. Tip in the chopped peppers and fry for a 3-4 mins to soften slightly. Add the dried pasta, chopped tomatoes, vegetable stock, tomato purée and 1/4 tsp each salt and ground black pepper. Bring to the boil, then turn the heat down to low and simmer with the lid on for 10-12 mins until the pasta is cooked.

Heat the oven to 220C/200C fan/gas 7. Remove the lid from the pan and gently stir in the tuna and spinach, breaking up any larger chunks of tuna. Continue to simmer briefly until the sauce is thickened and the spinach has wilted, 2-3 mins. Transfer to a large roasting dish (ours was 23cm x 33cm), top with the white sauce, then the cheese. Put in the oven for 15 mins until golden brown. Sprinkle with chopped chives and serve. If freezing, defrost in the fridge and reheat in the microwave.