Preheat the oven to 450F.

In a saucepan, heat the butter and oil until melted. Add the sliced onions and cook for 8-10 minutes until soft. Add the sugar, water, vinegar, and wine. Let it simmer until most of the liquid has evaporated. It should look dark and jammy. Season with salt and pepper.

In a bowl, combine the breadcrumbs, parsley, and garlic together. Season with salt and pepper.

To roast the bone marrow, line your baking sheet with foil and place a roasting rack on top.

Generously season the bone marrow with salt and roast for 8 minutes. Take it out and top each marrow with the breadcrumb mixture. Roast for another 7-8 minutes until it starts to pull away from the bone. Another way to check is to pierce the center of the marrow with fork. If it there is no resistance, Be sure not to let it go too long in the oven or it will just completely melt in the oven.

Serve immediately with the toasted bread, onion jam, and maldon on the side.