From The Cook's Companion where there is greater detail of additions and variations.
500g ripe tomatoes, cored, seeded and roughly sliced
1 onion, sliced
2 garlic cloves, sliced
1/2 cup extra virgin olive oil
black pepper, freshly ground
handful basil or oregano leaves, optional
Tumble tomato, onion and garlic with oil and put into a saucepan (or spread in a baking dish into which mixture will fit without too much extra space).
Cook over a moderate heat for about 15 minutes, (or bake at 180C tightly covered with foil, for at least one hour until tomato has collapsed, skins are wrinkled and golden-brown, and juices are flowing).
When tomato and onion are soft, press everything through the coarsest disc of a food mill. Season to taste and add a handful of herbs if you wish.