Melt butter in 2-quart saucepan over medium heat. Cook onion in butter about 2 minutes, stirring occasionally, until tender. Stir in flour, salt and pepper. Cook, stirring constantly, until bubbly; remove from heat. Stir in milk and 1 1/2 cups of the cheese. Heat to boiling, stirring constantly. Boil and stir 1 minute.
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