Peel the onion and cut it in half. Use a sharp knife or mandolin slicer and cut it into thin strips. I prefer to slice mine pole to pole instead of orbitally but you do what you like!
Add the water, vinegar, salt and sweetener to a glass and whisk until the salt and sugar dissolves. Alternatively you can bring this mixture to a boil if you prefer. Just make sure you let it cool down to room temp before covering with a lid.
Add the onions to a large mason jar (32 oz) or similar container. Toss in the garlic and black peppercorns as well. You can add any other flavorings you like! Pour the mixture over the onions, making sure they are submerged. Cover with a lid (unless the mixture is hot, then let it cool first)
Optionally, you can add 1 - 2 slices of raw beet to ensure the onions take on a vivid pink color. Remove the beets after an hour or so if you are worried about a beet taste coming through.
They are honestly pretty good after 30 minutes, but let them sit overnight in the fridge. They will get better after a few days. They are best the fresher they are, but are good for at least 2 - 3 weeks.