Preheat oven to 180°C/350°F. Lightly grease a medium-sized baking dish.
Place a saucepan or pot of water over a high heat and bring to a boil. Season with salt and cook the pasta for 3 minutes less than the instructions. Remove and drain.
Add the butter to a large saucepan over medium-high heat, and let it melt. Add onion, garlic, and sauté for 2 minutes, mixing regularly or until the onion softens. Add the flour and mix well. Cook for 2 minutes or until thickened and starts to bubble. Don’t let it brown. Slowly add the warmed milk in batches mixing between each batch until the mixture has absorbed all the milk. Once mixed, add in the cream and cook for 1 minute. Check the seasoning and remove it from the stovetop.
Add the drained pasta, tuna, and lemon zest to the sauce and mix well until combined. Transfer the mixture to the greased baking dish and spread evenly. Sprinkle with the Panko bread crumbs and cheese mixture evenly distributing the two, and bake in the oven for 30-35 minutes or until golden on top. Remove and allow to rest for 10 minutes before serving.
Serve on plates or bowls, garnish with cracked black pepper and flat leaf parsley (optional). Dig in.