Trim and slice the leeks, giving them a good wash in a colander to get rid of any grit. Peel and finely slice the onions and garlic. Place a large saucepan or casserole on a medium-low heat and add the olive oil and half of the butter, and all the sliced vegetables. Season with sea salt and ½ teaspoon of freshly ground black pepper. Sauté for 30 minutes, stirring occasionally, until everything has cooked down and is sticky and golden in colour.