To a small saucepan, add apple cider vinegar, water, honey, and salt. Bring to a simmer.
Add in mustard seeds and bay leaf. Reduce heat to low and simmer for 30 minutes.
Add in shallots and stir to combine. Place mixture in a glass jar and let sit at room temperature for 30 minutes.
Place lid on jar and refrigerate until ready to use.
Bone Marrow:
Using a bone saw, cut leg bones into pieces. Cut the ends of the bones off right where the marrow starts (see pictures in recipe post for reference). Then, cut the marrow bone into 4 equal pieces, about 2-3" long. Repeat with the other bone.
Rinse the bone dust off of the bones and let them dry on a wire rack in the fridge or wipe them with a paper towel until they're dry.
Preheat oven to 450°F. Line a roasting pan or sheet pan with parchment paper.
Season tops and bottoms of bones generously with salt. Place bones in roasting pan, upright.
Roast for 15-25 minutes, flipping halfway. To test for doneness, use a toothpick to poke all the way through the marrow. If it's harder to push the toothpick through the middle than it is through the ends, it is not done yet. You should be able to poke the toothpick through the whole piece of marrow with ease. The marrow should be bubbling but not completely melted.
Serve marrow with toasted baguette slices and pickled mustard seeds. Scoop out a small amount of marrow and spread on baguette. Top with pickled mustard seeds. Enjoy!
Notes
Elk bone marrow can be waxy and unpalatable to some. If your elk had some fat on the meat, cook some up and try it. If it's not waxy and you enjoy the flavor, you're likely to enjoy the marrow as well.
Don't toss the bone ends or the roasted marrow bones once you're done with them. Use them to make a batch of stock instead!
Elk bone marrow is best eaten hot. If you don't plan to eat them all right away, keep them in a warm oven to keep them hot. Get one bone out of the oven at a time to serve.