Peel the red onion and slice it thinly. You can use a sharp knife or a mandoline slicer for even cuts.

Pack the onion slices tightly into a clean glass jar or heat proof container, along with the jalapeño slices, peppercorns, mustard seeds, red pepper flakes, and bay leaf.

In a small pot, combine the apple cider vinegar, water, sugar, and salt. Heat the mixture over medium heat, stirring until the sugar and salt dissolve completely. This should take about 2-3 minutes.

Once dissolved, remove the vinegar mixture from the heat and immediately pour it over the onions and aromatics. Make sure the onions are fully submerged in the liquid.

Gently press the onions down with the back of a spoon if needed, to ensure everything is covered.

Allow the jar to cool to room temperature, uncovered.

Once cool, seal the jar with a lid and refrigerate. The onions will start to take on flavor in about 30 minutes, but they taste even better after a few hours.

Store in the refrigerator for up to 2 weeks, and enjoy as a topping for tacos, sandwiches, salads, or burgers. Enjoy!