Fill a bowl with ice water. Remove the marrow from the bones with a spoon. If there are veins in the marrow, remove them and discard. Place the marrow in water. Cover and refrigerate overnight. This will get rid of any blood or impurities and firm up the marrow. Scrape the bones clean with a spoon. Place them in a pot of boiling water and boil for 5-10 minutes. Remove the bones drain, pat dry and reserve.

Place a skillet over high heat. When it is hot, add bacon and cook until crisp and golden. Don’t’ let it burn; you don’t want any brown bits in the bottom of the pan. Let the bacon cool for 3-4 minutes.

Drain the marrow and pat it dry. Place the marrow in the bowl of a food processor and add 3 tablespoons of the bacon bits. Pulse to combine. Start the food processor and slowly add 3 tablespoons of the bacon fat. Process until you have a smooth mousse-like mixture; it will have the consistency of peanut butter. Once you have a smooth, uniform texture, stop the machine immediately. Scrape the mixture into a container and refrigerate until it is firm, at least 1 hour and up to 3 weeks.

Wash and dry the bowl of the food processor. Place the bread in the food processor and pulse for 30 seconds, until you have nice coarse crumbs. Add the parsley leaves, salt and pepper and pulse until the parsley is incorporated. Add the rest of the bacon and its fat and process, stopping the machine to scrape down the sides of the container occasionally. You want a uniform, moist mixture. Transfer the mixture to a container on the side.

When ready to serve, remove the bone marrow mousse from the refrigerator and let it come to room temperature; you want it to have the consistency of a stick of butter. Preheat the broiler to high and position the rack in the upper third of the oven. Spoon the bone marrow mouse into the bone cavity; overfill it just a little. Smooth the top of the mousse with an offset spatula so it is even with the bone, pack down the ends, and clean any excess mousse from the bone. Sprinkle bread crumbs over the surface of the mousse, covering it completely. Gently pat down the crumbs.

Fill a baking pan large enough to hold the bones in a single layer with rock or kosher salt and moisten the salt to form a bed for the bones. Place the bones on the salt and broil it until the bread crumbs are lightly toasted, 3-4 minutes. Check the bones after 90 seconds to make sure they’re not browning too quickly. Some of the marrow will run; that’s okay.

To serve, on each plate build a pile of rock or kosher salt. Nest the bones (or bone) in the salt. In a small bowl, toss the celery leaves with olive oil. Garnish each serving with 3-5 celery leaves and sprinkle of sea salt. Garnish with as much caviar as your taste and budget allow.