Make the red wine sauce. Cook the onions in the butter and oil until golden brown. Add the rest of the ingredients and simmer the sauce until it is reduced by half. Strain the sauce and keep warm.
Place the marrow bones in a roasting tray and scatter with the herbs. Season with salt and ground black pepper. Bake in the oven for about 30 minutes.
Serve the marrow bones with some red wine sauce spooned over and garnish with fresh herbs. Serve plenty of toasted bread on the side.