PREP: Wash hot red peppers under cold running water; drain. Peel onions; measure 4 quarts peeled onions. Put onions in a large bowl and sprinkle with salt. Pour cold water over onions just to cover. Cover; let stand 12 to 18 hours in refrigerator. Drain onions. Rinse onions under cold running water; drain. Cut a small slit in each hot pepper on two opposite sides.
COOK: Combine sugar, mustard seed, horseradish, and vinegar in a large saucepan. Bring mixture to a boil. Reduce heat to a simmer (180 degrees F); simmer 15 minutes.
FILL: Pack onions into a hot jar, leaving 1/2 inch headspace. Put 1/2 hot red pepper and 1/2 bay leaf into half-pint or 1 hot red pepper and 1 bay leaf in pint jar. Add 1/16 teaspoon Pickle Crisp to pint jar, if desired. Ladle hot pickling liquid over onions, leaving 1/2 inch headspace. Remove air bubbles. Clean jar rim. Center lid on jar and adjust band to fingertip-tight. Place jar on the rack elevated over simmering water (180 degrees F) in boiling-water canner. Repeat until all jars are filled.
PROCESS: Lower the rack into simmering water. Water must cover jars by 1 inch. Adjust heat to medium-high, cover canner and bring water to a rolling boil. Process half-pint or pint jars 10 minutes. Turn off heat and remove cover. Let jars cool 5 minutes. Remove jars from canner; do not retighten bands if loose. Cool 12 hours. Check seals. Label and store jars.
Note: When cutting or seeding hot peppers, wear rubber gloves to prevent hands from being burned.