Trim the top off each red onion, leaving the root end intact, then peel away the skin. Using either a very sharp knife or a mandoline set to its finest setting, slice the onions paper‐thin for maximum texture.

Arrange the delicate slices into one quart jar or two pint jars, gently layering them so they fit snugly.

Combine the vinegar, water, sugar (or maple syrup) and kosher salt in a small saucepan. Warm the mixture over medium heat until it steams and the sugar and salt have melted – about 2 minutes. (You can also microwave in short intervals.)

Evenly pour the steamy pickling liquid over the onions. Press them lightly to ensure every slice is fully immersed. Let the jar stand at room temperature for at least 1 hour. For more developed flavor and a bright pink hue, refrigerate for 12 hours (or longer).