Heat the butter in a frying pan over a medium heat. When the butter is foaming, add the caraway seeds and fry for 1-2 minutes, or until fragrant.
Add the flour and whisk to a smooth paste. Continue to cook, stirring vigorously, for a further 3-4 minutes, then pour in 125ml/4½fl oz of the milk. Whisk the milk and flour mixture together to a smooth paste.
When the mixture is bubbling, add another 125ml/4½fl oz of milk and whisk until it is bubbling and incorporated into the mixture.
Repeat the process with the remaining 250ml/9fl oz of milk. Continue to whisk and simmer until the sauce is smooth and thick enough to coat the back of a spoon. Remove the pan from the heat and stir in the cream cheese. Season, to taste, with salt and freshly ground black pepper.
Add the onion, tuna and cooked pasta to the cheese sauce and stir until well combined.
Pour the mixture into an ovenproof dish. Sprinkle over the breadcrumbs and lay the anchovy fillets on top. Bake in the oven for 30 minutes, or until the breadcrumbs are crisp and golden-brown and the sauce is bubbling.