Heat oven to
350F (175C). Butter 6-cup (1.5-Liter) casserole. Cook sliced potatoes in
boiling water 10 minutes. Drain immediately. Drain salmon, saving liquid.
Break salmon into bite-size pieces. Put liquid in cup and add enough milk
to make the 2 cups (500ml) liquid called for. Heat butter in medium
saucepan. Add flour, salt and pepper and stir to blend. Remove from heat
and add the 2 cups (500ml) liquid all at once. Stir to blend. Stir in
mustard. Return to moderate heat and cook until boiling, thickened and
smooth, stirring constantly. Turn heat to low and continue cooking and
stirring 2 minutes longer.
Put a layer of
1/4 of the potatoes in the prepared casserole. Top with 1/3 of the salmon,
1/3 of the onions and about 1/4 of the sauce. Repeat these layers twice
more and end with a final layer of potatoes and sauce. Bake 45 minutes or
until potatoes are tender and sauce is bubbling well.