DONE THE DAY BEFORE
Stand the fennel bulbs on their bases and cut vertically in half. Cut out the core and discard any fibrous outer segments. Slice thinly and place in a large bowl, sprinkle with salt and mix to combine. Cover bowl and refrigerate overnight.
In a large saucepan add the vinegar and sugar over high heat until the sugar dissolves. Bring to the boil then reduce to a simmer and cook, uncovered for 30 minutes. Do not stir, just leave to reduce slightly. Allow to cool, then also place in the fridge to cool completely overnight.
Squeeze out any extra liquid from the fennel and place in an airtight jar. Pour over pickling liquid to cover. Add thinly sliced garlic and whole chillies. Seal and refrigerate for 2 days to allow flavours to develop.
This pickled fennel will keep for up to 2 weeks in the fridge.