Marrow bones. You can either carve them off a beef shank, or special order them from your butcher. Either way, you shouldn't have to pay much for them. My recipe is modified from Bones.
4 shank bonesYep, that's it. Simplest. Recipe. Ever.
In a small container, cover the bones with water. Add 2 tsp of salt. Soak for 12 to 24 hours in the fridge.
Remove your brined (and what looks like bleached) bones from the water, pat them dry and place them on an oiled cookie sheet.
Pre-heat your oven to 450°F. Bake the bones in the oven for 15 to 25 minutes. Until the marrow is bubbling slightly and it leaves goo on a tester inserted all the way through.
Spread the roasted marrow on a piece of toasted homemade bread. Sprinkle with fleur de sel. Serve.
You're really gonna want to serve this in a dimly lit room. It's, um, kinda grotey lookin'. But rich and delicious. I mentioned bone marrow to a colleague at work today, and he said, "Yeah. Bone marrow is rich and lusty." Well, I wouldn't say it that way. But I guess I don't disagree.