In a medium saucepan, combine the distilled white vinegar, water, sugar, and kosher salt. Bring to a boil over medium-high heat, stirring frequently until the sugar and salt fully dissolve. Remove from heat.

While the brine heats, slice the red onions and garlic. Peel the onions, cut each in half from root to stem, and use a sharp knife or mandoline to slice each half ⅛-inch thick. Thinly slice the garlic cloves.

Pack the sliced onions, sliced garlic, and herb sprigs into a clean 16-ounce mason jar, layering as you go.

Carefully pour the hot brine over the onions until they're completely covered. Use a clean spoon to gently press the onions down so they stay fully submerged in the brine.

Let the jar cool to room temperature on the counter, about 30 minutes. Seal it tightly with a lid and refrigerate for at least 2 hours before serving. Stored properly in a sealed jar, these keep for up to 3 weeks in the refrigerator.