Half-fill a large saucepan with water and bring it to the boil. Add the pasta, return the water to the boil and cook according to the packet instructions until just tender. Just before the end of the cooking time, add the frozen broccoli florets to the pan and cook with the pasta for the final 2-3 minutes, until just tender.
Meanwhile, heat the butter, flour and milk in a saucepan over a medium heat until the mixture is just bubbling, whisking constantly with a balloon whisk until smooth and thickened to a roux.
Continue to cook the roux for a further 2-3 minutes, stirring constantly until it has thickened enough to coat the back of a spoon. Stir in half of the cheese until melted, then season, to taste, with salt and freshly ground black pepper.
Preheat the grill to its highest setting.
Drain the pasta and broccoli in a large colander and transfer to a pre-warmed shallow ovenproof dish (a lasagne dish is ideal). Flake the tuna on top of the pasta and broccoli.
Pour the cheese sauce over the pasta mixture, then top with the sliced tomatoes, if using. Sprinkle over the remaining cheese.
Place the pasta bake under the hot grill for 4-5 minutes, or until the cheese melts and starts to turn golden-brown. Serve immediately.