Sauté the Base: In a saucepan over medium heat, warm the olive oil and cook the finely chopped onion until soft and translucent. Add the minced garlic and cook for an additional minute until fragrant.
Build the Sauce: Stir in the tomato paste and passata, then add the apple cider vinegar, molasses, Worcestershire sauce, smoked paprika, mustard powder, and chilli flakes if using. Season with salt and freshly ground black pepper.
Simmer: Allow the sauce to simmer gently for 15 to 20 minutes, stirring occasionally, until it thickens slightly and the flavours meld together.
Finish and Store: Remove the sauce from heat and let it cool to room temperature. Transfer to an airtight container and refrigerate for up to a week.