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Preheat the oven to 375°F (190°C).
Boil the sliced potatoes in salted water for about 8-10 minutes until just tender but not falling apart. Drain and set aside.
In a saucepan, melt butter over medium heat. Add the sliced onions and cook until soft and translucent, about 5 minutes.
Stir in the flour and cook for 1-2 minutes, stirring constantly to make a roux.
Gradually whisk in the heavy cream (or milk) and broth, continuing to stir until the sauce thickens, about 5 minutes. Season with salt and pepper to taste.
Remove from heat and stir in half of the shredded cheese until melted and smooth.
In a baking dish, layer half of the boiled potatoes, then half of the tuna flakes. Pour half of the cream sauce over the layers. Repeat with remaining potatoes, tuna, and sauce.
Sprinkle the remaining cheese evenly on top.
Bake uncovered for 25-30 minutes, or until the top is golden and bubbly.
Remove from oven, let cool slightly, and garnish with chopped fresh parsley if desired before serving.
Serving and storage tips:
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