Place diced vegetables in a large bowl and season with salt, tossing to combine. Cover and let sit at room temperature overnight.
Set a colander into a large bowl and strain vegetables, reserving the liquid.
Heat a cast-iron skillet over medium and toast mustard seed, fennel seed, turmeric, mace blades (or nutmeg), and black peppercorns for 2–3 minutes or until fragrant, tossing frequently.
In a medium saucepan, bring vinegar and 2 cups of reserved vegetable liquid to a boil. Add sugar, stirring to dissolve, and toasted spices. Reduce heat to medium-low and reduce liquid slightly, about 4–5 minutes. Pour hot liquid over diced vegetables.
Let cool to room temperature, place in an airtight container, and refrigerate for up to 5 days. Or, to preserve, ladle mixture into sterilized jars kept warm in hot water and preserve according to standard water-bath technique. For more information about canning, see Ball’s freshpreserving.com.