Place the onions and serrano pepper into a medium-sized bowl. Set aside.

In a small sauce pan over medium-high heat, bring the vinegar, honey, salt, mustard seeds and bay leaf to a boil. Once the honey and salt are dissolved, stir the brine once or twice and immediately pour over the onions. Stir to combine.

Use the back of a spatula or spoon to press down upon the onions to fully submerge under the brine. Let the onions sit at room temperature until the onions have wilted, about one to two hours. Serve or cover and place into the fridge to chill until ready to serve.