In a medium saucepan over medium heat combine the apple cider vinegar, salt, and sugar. Heat until the salt and sugar dissolve, then remove from the heat.
Slice the red onions very thin using a chef's knife or mandoline. Put in a large bowl or jar.
Add the black peppercorns and sliced peppers to the bowl and toss.
Pour the hot vinegar mixture over the onion mixture. Allow to sit for 30 minutes before serving.
These can be kept in the refrigerator for up to three weeks. These can be frozen in portions in freezer bags for up to three months.
Notes
If you don't want to bring the vinegar mixture to a boil, you can add everything to the onions in a jar.Heating the vinegar mixture softens the onions and allows the vinegar mixture to penetrate quickly. It also allows you to keep them longer.You can add garlic, hot red pepper flakes, and any fresh herbs for different flavors.