To a large dutch oven over medium-high heat, add the oil. Season the beef generously with kosher salt and pepper. Working in batches in a single layer on the bottom of the pan, sear the meat until browned on all sides. Transfer the seared beef to a plate to rest while repeating with the remaining batches.
When all of the beef has been seared and set aside, add the carrots and onion to the pan. Cook, stirring occasionally, about 3 minutes. Add the garlic and mushrooms to the pan and continue cooking another 2 minutes. Add the potatoes and rosemary. Degalze the bottom of the pan with the red wine, scraping the bottom of the pan with a spatula. Stir in the tomato paste, dijon mustard, thyme, and bay leaf.
Transfer the contents of the pot to the crockpot. Pour in enough beef broth to cover, about 6 cups. Stir in the barley. Cover and cook on high for four hours. Reduce the crockpot temperature to low and cook two more hours. Discard the rosemary sprig and bay leaf. Season the stew with salt and pepper as needed. Serve with freshly chopped parsley or chives on top.