Preheat the oven to 180°C. Cook the pasta according to the directions on the package. Drain, rinse and drain again; return to the saucepan and cover.
Meanwhile, heat the oil over medium heat in a large saucepan. Add the onion, green capsicum and celery and cook for 5 minutes, or until the vegetables are tender. Stir in the flour and black pepper. Add the milk and cook, stirring constantly, for 10 minutes, or until the mixture is thickened and bubbly. Add the Swiss cheese, stirring until melted. Stir in the cooked pasta and then carefully stir in the tuna.
Spoon the mixture into a lightly greased 20 x 20 x 5 cm baking dish. Top with grated Parmesan cheese and bake for 25 minutes, or until heated through.