Preserving Method: Water Bath Canning

1. Combine mustard seeds and vinegar in 1-quart ball jar, cover and allow to stand on counter for 24 hours.  
2. After 24 hours, prepare boiling water canner. Heat jars in simmering water until ready for use. Do not boil. Wash lids in warm soapy water and set aside with bands.
3. Transfer mustard seeds to food processor or blender, add salt and puree until desired smoothness, adding water to thin, as needed.  Transfer to a 4-quart saucepan and stir in sweet pickle relish, adding more water if desired, to thin out to a consistency slightly thinner than ketchup (mustard will continue to thicken on the pantry shelf).  Bring to a boil over medium heat, stirring frequently, to prevent burning.
4. Ladle hot mustard into a hot jar, leaving 1⁄4-inch headspace. Wipe jar rim. Center lid on jar. Apply band, and adjust to fingertip-tight. Place jar in boiling-water canner. Repeat until all jars are filled.
5. Process jars 15 minutes, adjusting for altitude. Turn off heat; remove lid, and let jars stand 5 minutes. Remove jars and cool.