Bring water, apple cider vinegar, sugar, kosher salt, peppercorns, chile de arbol, and garlic to a simmer in a medium saucepan. Stir until sugar and salt dissolve.
Place sliced red onions in a heat safe jar. Pour brine over onions until submerged. Press onions down.
Let rest at room temperature for 30 minutes. Then use or cover and refrigerate.
Store in the refrigerator for up to 1 week if not using right away.