In a saucepan, add vinegar, brown sugar, black peppercorns and red chili. Let it simmer for 5-7 minutes on medium heat.
Meanwhile, sprinkle some salt over the shallots and let them sit for 5 minutes.
Transfer the salted shallots into a clean glass jar and top with the hot brine.
Let it cool down completely, top with a lid and let it sit for 3-4 hours before refrigerating.
Notes
Make sure you let the shallots rest in the brine for at least 24 hours to soak up all the brine and soften slightly.
If you want them to pickle quickly, slice them thinly using a sharp knife or a mandolin slicer and then pickle them. I actually did the same while making my pickled shredded carrots.
You can also follow the exact recipe and use sliced onions instead of shallots. Just remember that onions have a much stronger, pungent flavor as opposed to shallots, which are sweeter, so they may need an extra day in the refrigerator to mellow down.
I used a 1:1 proportion of vinegar and water to make the brine. You can go down to 1:2 ratio too if you want a more subtle flavor.
To bring a bit of citrusy zing to the pickle, reduce the amount of vinegar you're using and add some fresh lime juice instead. I used lime juice to make my pickled green chilies and it was AMAZING!