Heat the olive oil over medium-low heat in a large saucepan. Add the onions and salt and continue to cook, slowly, until they start to caramelize and turn an amber color, approximately 20 minutes.
Add the sugar, vinegar and thyme leaves, stirring to combine until sugars dissolve. Keep heat low, stirring occasionally for about 15 minutes until the mixture reduces and thickens.
Remove from heat and serve or allow to cool fully before refrigerating.
If you've tried this recipe, come back and let us know how it was in the comments or star ratings!