Prepare the onions: Peel and thinly slice the onions evenly.
Cook slowly: Heat butter or oil in a large pan over low–medium heat. Add onions and a pinch of salt. Stir occasionally.
Add sugar (optional): Sprinkle sugar over onions to help with caramelisation.
Simmer gently: Add a little water or stock to prevent sticking. Cover and cook for 20–30 minutes, stirring occasionally, until onions are soft, golden, and sweet.
Finish with seasoning: Stir in vinegar or lemon juice if using. Adjust salt and pepper. Add herbs if desired.
Serve warm: Slaphakskeentjies are ready to accompany stews, braaied meats, or pap.