Melt the butter in a saucepan over medium heat.
Add onion. Cook and stir until tender. Stir in
the flour until well blended. Gradually stir in
the milk so that no lumps form. Season with
salt, pepper, poultry seasoning, and mustard.
Stir mixture constantly until it thickens.
Remove from heat. Add 2/3 c. cheese, stirring
until melted. Add mushrooms.
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