Heat the oven to 220C (200C fan)/425F/gas 7. Put three tablespoons of oil into a large, ovenproof saute pan for which you have a lid, and heat on a medium-high flame. Add the onion and cook, stirring occasionally, for about eight minutes, or until softened and browned. Add the garlic, chilli and anchovies, cook for a minute more, until fragrant, then stir in the capers, half the lemon, 45g of the olives, tuna, tomato paste, tinned tomatoes, orzo, 450ml water, a teaspoon of salt and a generous grind of pepper. Bring to a simmer, then cover and transfer to the oven for 20 minutes, or until the orzo is cooked through.
Turn up the oven to 230C (210C fan)/450F/gas 8. Lift the lid off the pan, then haphazardly top the mixture with the sliced tomatoes, sprinkle over the cheese and return to the oven, uncovered, for 10-12 minutes, or until bubbling and lightly browned. Remove, leave to rest for about 10 minutes, then top with the remaining olives and preserved lemon, the basil and the remaining tablespoon of oil, and serve straight from the pot.