Preheat the oven to 350 degrees Fahrenheit with your pizza stone inside. Line a baking sheet with parchment paper.
Put the 1/2 cup apple cider vinegar, 1 cup water, 1 tablespoon sugar, and 1/2 teaspoon salt in a small, flat pan and bring to a simmer over medium heat. Stir to dissolve the sugar and salt.
Trim the top and bottom off the red onion, then peel off and discard the outer layer. Cut the onion into slices.
Once the vinegar mixture has come to a simmer, remove it from the heat and put the onion slices into the hot vinegar. Set aside for 20-30 minutes, or until you’re ready to make your pizzas.