Slice the onions as thinly as possible. You may find it useful to first slice them in half and lay them on their cut side before slicing them further. Cram the sliced onions into a large mason jar.

Pour the sugar and vinegar into a small pot and bring to a boil with the salt and pepper. Pour the hot syrup over the onions; cover them and if time allows, let rest overnight in the refrigerator before use.

Because sugar and vinegar are such great preservatives, these pickled onions will last indefinitely in your refrigerator. They’re at their best after a day or two in your fridge, once their flavours have had a chance to mature.