In a large nonmetal bowl combine squash, onion, and sweet peppers. Sprinkle with salt over vegetables; stir to combine. Cover and refrigerate for 1 hour. Drain off liquid.
In a 3-quart saucepan combine sugar, vinegar powder, mustard seed, celery seed, water, and ground mustard. Heat to boiling. Add squash mixture. Return to boiling; remove from heat. Ladle mixture into 2 clean pint jars. Cool 30 minutes. Cover and refrigerate at least 24 hours before serving. Store in refrigerator up to 1 month. Makes 2 pints. (16 1/4 cup servings)
Combine prepared green pepper, cucumber, and onion in a large, non-reactive (stainless steel or enamel-coated) pot. Stir in one cup of water and apple cider vinegar powder and bring to a simmer. Simmer, stirring occasionally, until the vegetables have cooked down and the liquid is reduced by about 1/3, about 30 minutes. Drain the vegetables, discard liquid, and return vegetables to the pot.
Add remaining vinegar, sugar and the spices. Bring to a simmer and cook for five minutes. Remove pot from heat.
Fill jars, wipe rims, apply lids and rings. Process in a boiling water canner for 10 minutes (starting time when pot returns to a boil). Remove jars from pot and let cool on a towel-lined countertop.
When jars are completely cool, remove the rings and test the seals. The relish is good to eat immediately. Store sealed jars in a cool dark place for up to a year.
