Start by caramelising the onions.  To do this get a pan and add onions and oil. Cook them on a low heat for about an hour.

Season the exposed marrow bones and bake in the oven ( (200°C/400°F/ Gas 6) until the marrow is soft (around 8 mins).

Top the marrow with the onions and place under the grill.

Grill until singed on top, sprinkle with sea salt and serve with toasted bread.