Mix vegetables together in a medium bowl.
In a small bowl, combine the salt, dill, apple cider vinegar, sugar, whey, and mustard seeds.
Pour the veggies into a quart-sized, wide-mouth Mason jar, pushing down as necessary (a sauerkraut pounder is great for this!). Vegetables should be about one inch below the mouth of the jar.
Pour the liquid mixture over the vegetables in the jar.
Place a fermentation weight or smaller lid on top of your vegetables to keep them under the brine.
Lightly screw a lid onto the jar or add a fermentation lid.
Ferment the relish on your countertop for three days. If not using a fermentation lid, burp your relish once a day to prevent gasses from building up in the jar.
After three days, taste your relish. If the flavor is to your liking, place it in the refrigerator. If you would like to ferment it more, leave it on the counter for an additional 24 hours.
Store your relish in the refrigerator for up to 6 months.