Thinly slice the onion with a knife or mandoline, then place in a 16-ounce jar. Add any optional flavor boosters.
In a measuring cup, stir together the water, vinegar, honey, and salt.
Carefully pour the brine into the jar over the onions. Cover with a lid and refrigerate. They'll be ready to eat after about an hour in the fridge (they'll be vibrant pink), and will keep for 2 to 3 weeks in the fridge.
