Place potatoes into a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until tender, 10 to 15 minutes. Drain.
Mix reserved liquid from salmon and enough milk to make a total of 2 cups together in a measuring cup or bowl.
Preheat oven to 375 degrees F (190 degrees C). Generously grease a casserole dish.
Melt butter in a saucepan over medium-low heat. Stir flour into melted butter and cook until dissolved, about 1 minute. Gradually add milk mixture to butter-flour mixture; cook, stirring constantly, until sauce is thickened and smooth, 5 to 10 minutes.
Layer potatoes, salmon, and sauce, alternating 1 or 2 times, into the prepared casserole dish.
Whisk mayonnaise, Parmesan cheese, Worcestershire sauce, and mustard together in a bowl; pour over casserole.
Bake in the preheated oven until casserole is bubbling, about 30 minutes.
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