Mince the onions and cut the salmon into bite-sized pieces. Season the salmon with salt, pepper, and sake.
Pre-heat your oven to 445F/230C.
Sauté the salmon in a pan with 1 tablespoon of oil. Once golden brown, arrange in the bottom of the gratin plate.
Thoroughly cook the onions with butter (or margarine). They should be wilted and have a slightly amber color.
Add the flour and cook until no longer floury. Adjust the heat to avoid burning.
Add milk to the pan gradually to create the white sauce. Once it's boiled slightly and has a thickness, turn off the heat and season with salt and pepper.
We're going to use a lot of cheese for the flavor at the end, so if you feel it's a bit thin at this point, that's okay!
If using a store-bought white sauce, add to the pan in Step 4, and add milk accordingly to adjust the thickness.
Cover your gratin plate from Step 2 with the white sauce you prepared in Step 5. Cover with easy melting cheese, Parmesan cheese, and parsley, in that order.
Bake for about 10 minutes at 445F/230C. Once the cheese has a nice baked color, it's finished.
Story Behind this Recipe
This is one way to use cheap autumn salmon. I tried out various recipes but this gratin was well-received by my family, so it often makes an appearance during the fall months.
Be sure to thoroughly cook the fish in the pan. In the oven, it'll get a nice cooked look on the outside, but without any heat through the middle, it'll still be raw. So be sure to cook it well in Step 3!