Pat beef shank very dry and season with salt and pepper. Heat oil in a large Dutch oven or pot over medium-high heat and sear the shank until deeply browned, about 3-5 minutes per side. Set shank aside.
Reduce heat to medium and add butter, diced onion, and celery. Cook for 5-7 minutes, stirring occasionally, until softened and beginning to brown. Add minced garlic and cook an additional 30-60 seconds, or until fragrant.
Deglaze the pot by pouring in chicken broth and gently scraping the bottom to release all the browned bits. Add Worcestershire sauce and Marmite, stirring until Marmite dissolves. Then add in bay leaf, thyme, rosemary sprig, and black pepper.
Return beef shank and any accumulated juices to the pot. Bring to a gentle boil, then reduce to a low simmer and partially cover. Cook for 1 hour.
Stir in rinsed split peas, then continue simmering for 30 minutes. Dice or shred root vegetables, add to pot, and simmer another 30-45 minutes, or until vegetables and peas are tender and beef easily pulls away from the bone.
Remove beef shank and set aside until cool enough to handle. Remove meat from bone and shred or chop meat into bite-sized pieces and return to pot.
Remove and discard bay leaves and rosemary sprig. Add quick pearl barley and chopped cabbage. Simmer uncovered for about 10-15 minutes, or until barley is tender and cabbage is softened. Remove from heat and finish off with apple cider vinegar and salt to taste, if necessary.
Serve individual portions topped with fresh chopped parsley.
Notes
I chose to shred the turnip and dice the other root vegetables for the sake of textural variety, but you can choose to shred or dice all of them if you prefer. The reason I don't add them at the same time as the split peas is to ensure that they soften without getting too mushy.
Make sure to discard any tough pieces of connective tissue. If there's marrow in the bone, you can stir it into the broth, if desired.
Taking your time to brown the beef shank before adding the broth contributes a surprising amount of flavor.
Soaking the split peas before cooking isn't necessary, but don't forget to give them a good rinse.
If the beef doesn't pull away easily from the bone, it simply needs more time in the pot. Beef shank contains a lot of connective tissue, so sometimes it can still seem tough after an hour or two of cooking. The meat should be fork-tender and come right off when it's ready.
This Scotch broth recipe will likely thicken as it cools, as the barley and split peas tend to absorb liquid over time. Add additional broth to thin it out if desired.
Refrigerate in an airtight container for up to 3-4 days.
You can freeze this Scotch broth recipe for longer storage. For best texture, I recommend freezing individual portions and thawing them in the fridge overnight before serving, adding broth or water as needed.