In a small bowl, combine the first 5 ingredients; set aside.
In a shallow bowl, whisk flour and beer until smooth. Let stand 20 minutes. Cut onions into 1/4-in. slices; separate into rings. Dip onion rings in flour mixture.
In an electric skillet, heat 1 in. oil to 375°. Fry onion rings, a few at a time, until golden brown, 2-3 minutes on each side. Drain on paper towels. Serve immediately with Creole mustard sauce.
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