Wash a wide-mouth pint mason jar and and its lid in hot soapy water. Then rinse and let air dry.
Thinly slice the jalapeño peppers. (To make a milder heat level, you can remove some of the seeds as desired.) Fill the jar with the slices.
Peel the garlic cloves, leaving them whole. In a small saucepan, combine the garlic, vinegar, water, sugar, kosher salt, peppercorns and bay leaf. Bring to a boil, stirring occasionally to dissolve the sugar and salt (this will take only a few minutes).
Once boiling, pour the brine into the jar (including the bay leaf). Tap the jar on the counter to release any air bubbles. Discard any remaining brine, or top off the jar with extra water if any peppers are exposed at the top.
Screw on the lid tightly and allow to cool to room temperature (about 1 hour). Eat immediately, or refrigerate until serving. Keeps up to 1 month refrigerated.