Cut the onion and garlic into smaller pieces.
Turn on a pot to medium-high heat (9 out of 14 on my stove).
Add butter to the pot and let it melt.
Add the onion and garlic to the melted butter.
Let it cook for a couple of minutes. The onions should just become soft and not take on color.
Add the vinegar and let it boil for 2 minutes.
Then add the brown sugar and let it simmer further.
Grate the lemon zest (avoid the white part) and squeeze the juice. Add both to the sauce along with olive oil, tomato puree, passata, and chili.
Let it simmer for 20 - 30 minutes and season with salt, cayenne pepper, and cumin.
Blend everything together. Now the sauce is ready!