It is best to avoid frying with oil if at all possible as heat causes toxic changes in the structure of the oil.
- Cook onion in dry heavy pan until it starts to catch and the natural sugars start to brown (Le Creuset is good). There is no need for any oil. Add a couple of splashes of white wine, verjuice, beer or even water and heat until fragrant.
Red onions are particularly good done this way with balsamic vinegar as the liquid and a spoonful of brown sugar - caramelised onions!
If absolutely necessary, a small amount (1 -2 teaspoons) of extra virgin olive oil may be used, and a splash of water added to keep the temperature down.