• Peel the red onion and slice it about ⅛ inch thick using a mandoline or sharp knife. The thinner the onion is sliced, the more quickly it will soften and absorb the brine. Separate the slices and place them in a pint-size glass jar. Glass is nonreactive and will not react with the acidic vinegar brine as aluminum, copper, or unlined cast iron can.
• Combine the apple cider vinegar, water, your choice of sweetener, and kosher salt in a small saucepan. Warm over medium heat, stirring until the sweetener and salt are completely dissolved. The brine only needs to be warm; do not bring it to a boil.
• Slowly pour the warm brine over the sliced onions. Gently press the onions down until they are submerged. If all the onion will not fit beneath the brine, remove some rather than packing the jar too tightly.
• Leave the jar uncovered at room temperature for about 30 minutes, or until cooled. Cover the jar and refrigerate. The onions can be eaten after 30 minutes, but their flavor and color will continue to develop over the next several hours.

Notes

• Pure cane sugar gives the onions a clean, classic flavor. Brown sugar adds a deeper caramel-like flavor, while honey adds a mild floral sweetness.
• This is a refrigerator-pickle recipe and is not intended for shelf-stable canning.

Storage

• Store covered in the refrigerator for 2 to 4 weeks. Discard if the onions develop an unusual odor, visible mold, or other signs of spoilage.