Here's What You'll Need
red onion, rice vinegar, salt, granulated sugar, garlic, whole black peppercorns, thyme, lemon peel, red pepper flakes, mustard seeds
Pour vinegar in a jar that is large enough to fit the vinegar and the red onions. If the vinegar only comes up halfway on the jar, you've likely got enough room to add the onions.
Add salt, sugar, garlic, and peppercorns to the vinegar. If you're using any fresh herbs or citrus, add them now. Seal the container and shake to dissolve sugar and salt, then reopen.
Place thinly sliced red onion in a sieve or fine mesh strainer and set in a sink or over a glass bowl. Heat water in a kettle until boiling, then pour over sliced red onions.
Carefully transfer onions to the jar, then seal, shake, and place in a refrigerator for at least 30 minutes. Pickled onions will keep for 2-3 weeks.