Prepare the Potatoes : Preheat your oven to 180°C (350°F). Bring a large pot of salted water to a boil. Add the sliced potatoes and cook for 5 minutes, or until just tender. Drain and set aside.

Cook the Spinach : Heat a large skillet over medium heat. Add the chopped spinach and cook for 2-3 minutes, stirring occasionally, until wilted. Remove from heat and set aside.

Make the Sauce : In a medium saucepan, melt the butter over medium heat. Add the sliced onion and sauté until softened, about 3-4 minutes. Stir in the flour and cook for 1 minute, stirring constantly. Gradually whisk in the milk, ensuring no lumps form. Cook the mixture for 5-7 minutes, stirring frequently, until the sauce thickens. Remove from heat and stir in 1/2 cup Parmesan cheese and the lemon zest. Season with salt and pepper to taste.

Assemble the Bake : Grease a baking dish (approximately 9x13 inches). Layer half of the potato slices on the bottom of the dish. Top with half of the cooked spinach, half of the flaked tuna, and half of the creamy sauce. Repeat the layers with the remaining ingredients.

Add the Topping and Bake : Sprinkle the extra 1/4 cup Parmesan cheese evenly over the top. Bake in the preheated oven for 30-35 minutes, or until the top is golden and bubbling.

Serve : Remove the bake from the oven and let it rest for 5 minutes before serving. Enjoy this warm and comforting dish with a crisp side salad or crusty bread.