Bring 200ml water and 200ml white vinegar along with 1 tbsp each salt and sugar just to a boil in a saucepan and take off heat to slightly cool.
Pack 1/2 of a mandolined red onion and 1 tbsp black peppercorns into a clean 600ml wide-mouthed mason jar.
Slowly pour the still-steaming vinegar solution over the onions so the onions are completely covered but headspace remains in the jar, and lid.
Can be used after 20 minutes. Cool to room temperature on counter, then move to the fridge. Use within a month or so.