Slice onion into rounds about 1/4 inch thick or thinner. The thinner the slices the softer the onions will be.
OPTIONAL: Place slices in glass bowl and cover with ice cold water for 10 minutes to take out some of the sharp bite from the onions.
Drain onions and divide into two 1 cup (1/2 pint) jars. Stuff onions in.
In small saucepan mix vinegar, water, sugar, salt, allspice berries, peppercorns and hot pepper flakes in small saucepan. Cover and bring to boil, stirring to dissolve all ingredients.
Pour mixture over onions to cover fully.
Rest until cooled, about 1 hour.
Seal with lid and store in fridge. Let flavours infuse for two to three days for best flavour and texture, but can be eaten right away. Once opened, use within 3-4 weeks.
Notes
Note: When working with vinegar avoid bowls or pots made with aluminum or metal that may react with acid. Stainless steel and glass pots and bowls are best.Instead of dried hot pepper flakes, try adding a hot chili pepper to each jar.Love garlic - add a clove or two of that too.The vinegar makes this a highly acidic environment, so it's safe to experiment with seasoning ingredients like hot peppers and garlic. The sugar mellows out the acid but you can leave it out or swap in your favourite sweetener if you wish.Try other vinegars - rice wine vinegar, red wine vinegar would work well too