[header] How to substitute for mustard seed [title] Use prepared horseradish to add spice. [step] Prepared horseradish is made by combining ground horseradish with vinegar, and sometimes other spices, to form a sauce. You can get it from your local supermarket. 
In place of mustard seed, use the same amount of prepared horseradish to add spice and complexity to your recipe. [title] Add mayonnaise to achieve the same consistency.