Substitute cowboy steak with boneless ribeye steak or fillet on the bone.
Pre-heat the oven to 200°C/390°F and line a baking tray with tinfoil.
Coat the marrow bones in olive oil, generously season with salt and pepper. Arrange the bones on the baking tray cut side up and scatter over thyme sprigs.
Roast the marrow bones for 15 minutes or until the marrow starts to bubble around the edges.
In the meantime, lightly coat the ribeye steaks in olive oil and generously season with salt and freshly ground black pepper.
Grill the steaks on a hot fire or griddle pan, turning to cook on both sides to your preferred doneness; depending on the thickness of the steak, it will take approximately 5 to 7 minutes on each side for medium-rare.
Leave the meat to rest for half its cooking time before slicing to ensure it is moist, tender and juicy.
Make the sauce by scooping the marrow out from the bones into a pan. Add the butter, onion and garlic and sauté on a medium-low heat until soft.
Stir in the vinegar to deglaze the pan (loosen the flavourful bits that have stuck to the pan).
Pour in the beef stock and dijon mustard and allow the sauce to simmer and reduce for a few minutes or until it thickens and the marrow starts to melt. Remove from the heat.
Spoon the roasted bone marrow sauce over the cowboy steaks or serve on the side.