Finely chop or grind onions, tomatoes, and peppers.

Sprinkle with ¼ cup salt.

Allow to rest for 3 hours.

Drain vegetable mixture well.

Add vinegar and water to vegetables.

Bring to a boil and continue boiling for 15 minutes.

Add sugar, mustard, and turmeric.

Boil for 5 more minutes, stirring often.

Ladle into clean jars.

Process for 10 minutes in boiling-water bath.