Preheat oven to 350F (180C).

Melt a tablespoon of butter in a large skillet or saucepan over medium heat. Add the potato slices, 1/4 cup of water and cook for about 5 minutes, until they just begin to soften.

Meanwhile, in a separate pan, melt another tablespoon of butter over medium-high heat and fry the onions for another 5 minutes until they begin to soften.

Add the onions to the potatoes, season with salt and pepper and continue to cook for several more minutes.

Remove from the heat and let cool.

Meanwhile, prepare a white sauce by adding the flour, the milk, and 1 1/2 tablespoons of butter to a large pot over medium heat, stirring frequently. Add the cheese, and continue to cook until cheese melts and the flour dissolves. stirring frequently.  This should produce a creamy sauce.

Add the potato and onion mixture, and stir to coat.

Lightly grease a 9x9 inch baking pan. Roll out the puff pastry onto a floured surface, and the line the baking pan with the puff pastry.

Spoon the potato, onion and sauce mixture into the pan.

Bake at 350F (180C) for 40 minutes.

Let cool for several minutes before serving.