Heat the olive oil in a large saucepan or heavy - based flameproof casserole dish over medium heat and fry the onion for three to four minutes or until translucent. (womenshealthandfitness.com.au)
In a medium flameproof casserole dish, heat the oil and sauté the onions over medium heat for 4 minutes, or until they are soft. (humanesociety.org)
Heat the oil in a large flameproof casserole or saucepan and gently fry the onion and leeks for 10 - 15 mins until well softened, stirring occasionally. (bbcgoodfood.com)