Step 1: Prepare the onions. Thinly slice the red onion using a mandoline or a sharp knife. Aim for consistent, thin rings. Place the sliced onions in a heat-proof jar (a pint-sized Mason jar works well).
Step 2: Make an ice bath (optional but recommended). Fill a bowl with ice water. Submerge the sliced onions in the ice water for 10-15 minutes. This helps mellow the onion flavor and keeps them crisp. Drain thoroughly.
Step 3: Prepare the brine. In a small saucepan, combine the white vinegar, water, granulated sugar, and sea salt. Add any optional spices like peppercorns, bay leaf, or red pepper flakes. Bring the mixture to a boil over medium-high heat, stirring until the sugar and salt are fully dissolved.
Step 4: Combine and pickle. Pour the hot brine over the sliced onions in the jar, ensuring they are completely submerged. If needed, gently press the onions down with a spoon to keep them under the liquid.
Step 5: Cool and refrigerate. Let the jar cool to room temperature on your countertop. Once cool, cover tightly with a lid and transfer to the refrigerator. The pickled onions will be ready to eat in about 30 minutes, but the flavor is best after a few hours or overnight.
Step 6: Store. Store your quick pickled red onions in the refrigerator for up to 2-3 weeks. Enjoy the tangy crunch on your favorite dishes!