Heat a small frying pan on a medium setting and add a drizzle of olive oil. Sauté 30 g onion until soft and golden, then remove from the heat.
Transfer the onion to a blender and add all the other ingredients. Blend until smooth. (Alternatively, move onto the next step and use an immersion blender.)
Pour the sauce into a saucepan and bring to almost boiling over medium heat.
Once almost boiling, reduce the heat to medium-low and simmer for about 3 minutes. Stir occasionally.
Turn off the heat and allow to cool to your preferred temperature before serving. Transfer to a pourable container and enjoy with your favorite steak!