Choose your swap based on what you need most: tang (Dijon), bite (horseradish mustard), or graininess (homemade mustard seeds).
Use the listed ratio; if your substitute is sharper than whole grain mustard, start with slightly less and add more to taste.
For texture matching, whisk longer or pulse briefly (homemade mustard seeds) to create a coarse grain feel.
Thin with 1–2 teaspoons water or vinegar if the sauce becomes too thick; for baking, keep the swap smooth by stirring thoroughly.
Rest 2–3 minutes before serving so mustard flavor rounds out and bonds with the other ingredients.