Start by preheating your oven to 400°F (200°C). Bring a large pot of salted water to a boil and cook the pasta shells for 8–10 minutes until they’re just tender but still hold their shape. Drain them well in a colander and set aside.
In a large frying pan, heat the sunflower oil over medium heat. Add the fennel and onion, then cook gently for 3–5 minutes, stirring occasionally, until they’ve softened and become translucent but haven’t taken on any color. Remove from the heat and set aside.
In a large saucepan, melt the butter over medium heat. Once melted and foaming, sprinkle in the flour and stir continuously for 1 minute to create a smooth roux, cooking out the raw flour taste. Remove the pan from the heat and gradually pour in the milk, whisking constantly to prevent lumps from forming. Return the pan to the heat and bring the sauce to a gentle boil, stirring steadily until it thickens into a smooth, creamy consistency. Let it simmer for 2–3 minutes to fully develop the flavor and texture.
Fold the cooked pasta shells into the white sauce along with the softened fennel and onion mixture. Add the flaked tuna, chopped hard-boiled eggs, and grated cheese, stirring everything together until evenly combined. Season the mixture generously with salt and pepper to taste. Transfer the pasta mixture into a shallow ovenproof dish, spreading it out evenly.
Place the dish in the preheated oven and bake for approximately 30 minutes, or until the casserole is heated all the way through and the top has turned a beautiful golden brown. Remove from the oven and let it rest for a minute or two, then serve hot with a generous sprinkle of freshly chopped parsley scattered over the top for a fresh, colorful finish.
