Courtesy: Marisa McClellan (adapted from Food In Jars)
2 cups apple cider vinegar
3/4 cup granulated sugar
2 Tablespoons pickling salt
3 pounds red onions, trimmed and thinly sliced
2 teaspoons mustard seed
1 teaspoon celery seed
1/2 teaspoon red pepper flakes
Prepare a boiling water bath and 3 regular-mouth pint jars. Place the lids in a small saucepan, cover them with water, and simmer over very low heat.
Combine the vinegar, 1 1/2 cups water, sugar, and salt in a pot over high heat and bring the brine to a boil.
Add sliced onions to brine and stir to combine. Reduce heat to medium and simmer briefly to soften onions.
Meanwhile, combine the remaining spices in a small bowl and stir to blend. Add the spic blend to the sterilized jars, distributing evenly.
Using tongs, evenly divide the onions between the 3 jars. Pour hot brine into each jar, leaving 1/2-inch headspace. Gently tap the jars on a towel-lined countertop to help loosen any bubbles before using a wooden chopstick to dislodge any remaining bubbles. Check the headspace again and add more brine if necessary.
Wipe the rims, apply the lids and rings, and process in a boiling water bath for 10 minutes.
Let these pickles cure for at least 48 hours before eating.
Makes 3 1-pint jars