Microwave
In a large, microwave-safe bowl, combine the sliced red onions, lemon juice, and sea salt.
Cover loosely with a microwave-safe plate or lid.
Microwave on full power (800W) for 10 minutes, stopping to stir halfway through at the 5-minute mark.
Carefully remove the bowl, stir in the granulated sugar until dissolved, and return to the microwave.
Cook for another 15–20 minutes, stirring every 5 minutes to prevent it from catching.
The chutney is ready when most of the liquid has evaporated, it has a jammy consistency, and the colour has deepened to a rich purple. To test if it's set, place a small spoonful on a chilled plate; it should hold its shape without running.
Taste and adjust seasoning, adding more salt or sugar if needed. Allow to cool slightly before potting into a 500ml (16.9 fl oz) sterilised jar while both the chutney and jar are still warm. It will keep for a month in the fridge.
Stovetop Alternative
If you don't have a microwave, place all in a heavy-based pan.
Cover and cook over medium heat for 5 minutes to allow the onions to release their juices.
Remove the lid and continue to simmer, stirring occasionally, until the onions are very soft, sticky, and most of the liquid has evaporated.
Don't worry if it sticks a little; the caramelization adds flavour!
Add a splash of balsamic vinegar along with the sugar for a deeper, more complex flavour profile.
Let the chutney rest in the fridge for at least 24 hours before serving to allow the flavours to fully meld and develop.
