Heat 4 tablespoons of oil in a large pan over medium heat.
When the oil is hot, but not smoking, add the sliced onions. Give the mixture a good stir, so that the onion slices are evenly coated with the oil.
Leave undisturbed for 10-15 minutes, so that the onion slices in the bottom of the pan crisp up and brown slightly.
After that time, stir the mixture again and reduce the heatto low. Cook for additional 45 minutes.
Stir every 20 minutes and add 1 tablespoon of water each time (2 in total), to release some caramelized bits that might have stuck to the bottom of the pan.
In the meantime, grate the marrow into a large bowl.
Transfer the grated marrow to a colander and squeeze it with the back of a spoon (or using your hands) to release as much water as possible. Set aside until the onions are cooked.
As soon as the onions have caramelized, add the marrow, low salt, pepper and additional 2 tablespoons of olive oil to the pan.
Cook, stirring often, for additional 30 to 40 minutes, or until the marrow has reduced significantly and the mixture has a jam-y kind of texture.
The spread can be kept, stored in a airtight container in the fridge, for up to 1 week.