Add the sliced red onions to a 2-3 cup jar. If it's a tight fit, just use a wooden spoon to smush them down into the jar.
Add the sugar (1 tablespoon), kosher salt (1/2 tablespoon), and peppercorns (1/2 teaspoon) to the jar.
Pour white vinegar (3/4 to 1 cup) on top of the onions, stopping when you get halfway up the jar.
Pour boiling water (3/4 to 1 cup) into the jar. I like to microwave water in my pyrex measuring cup until it's boiling (about 1.5 minutes) and then pour it from there, but you can use a kettle or pot on the stovetop, too.
Cover the jar and shake until the sugar and salt are dissolved.
Refrigerate for about 30 minutes, or until they're cooled off. The taste will get better the longer they sit.
Storage: these pickled red onions will stay good for several weeks in the fridge in an airtight jar or container.
Variations: The peppercorns can be omitted if you don't have them, and/or other ingredients can be added to the jar such as a smashed clove of garlic or two, fresh herbs like thyme or oregano, other whole spices like coriander seeds, celery seed, or allspice berries, sliced fresh ginger, crushed red pepper for spice, etc. Play around with it!
Serving suggestions for pickled red onions: Top roasted veggies with them, use them on sandwiches or tacos, add them to a bagel with cream cheese, add to salads or pastas, top deviled eggs with them, add them to a cheese board... or eat them straight out of the jar!
You can use the brine in salad dressings, stirred into soups, or in other recipes that require an acidic ingredient.