Add something sweet. Things you can use to add sweetness include honey, maple syrup, and fruit jams.
Add something rich and fatty. Adding a dairy product to your dish will help neutralize too much heat as well as weaken the taste of mustard. If dairy isn’t your thing, you can try adding some olive oil. Or avocado.
Add an acidic ingredient. Lime juice, lemon juice, and a splash of vinegar can all help. If wine would suit your dish, this can also help, but make sure you let it cook-off.
Add something starchy. Starchy foods (potatoes, rice, beans) help offset intense flavors like mustard by adding more ‘blandness’ to the dish.
Dilute a sauce with water or stock. A quick and dirty solution for a sauce that’s got too much mustard in is to dilute it with some water or stock. Of course, this won’t work for non-liquid dishes so skip ahead if you’ve got a potato salad problem.
Simmer the sauce for longer. If you’re making a sauce the requires cooking, sometimes all it needs is a bit more time. As sauces cook, their flavors naturally mellow and you might find that after an extra 20 minutes, the mustard isn’t as strong.
Rinse the sauce off (good for potato/pasta salad). This one might sound a little crazy, but if you have something like a potato salad where the sauce is coating a relatively sturdy ingredient, you can put the whole thing in a colander and rinse the sauce off.