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What recipe are you looking to make the substitution in? Or are you asking a general question? I really depends on what you are making. Sometimes you can just omit it entirely but the flavor will be altered.
Agree with sdebrango about needing to know what recipe. Suspect in some cases an acid would help just to replace some of the oomph from mustard.
What about horseradish? Are you allergic to that? I agree with above it depends on the recipe.
I don't have a particular recipe at the moment. I usually omit the mustard when cooking but wonder if the food is more bland than it needs to be. If mustard is the main element in the recipe, I usually omit. I might try horseradish though.
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