Place the onion, sugar and lemon juice in a saucepan. Heat over medium-heat while stirring to melt the sugar.
Cook for about 5 - 10 minutes until the liquid thickens and has a syrup-like consistency.
Add the salt, crushed chili flakes and cubed peaches. Stir to mix.
Heat over medium-heat while stirring frequently until the peaches soften, and the liquid is mostly evaporated. This may take about 20 - 30 minutes.
Scrape the relish into glass jars and serve as needed (can be stored in the fridge for about 5 - 7 days).
