Heat the vegetable oil in a large skillet over medium-high heat. Add the beef stew meat in batches and brown on all sides, then transfer the meat to the slow cooker.
Add the diced onion, minced garlic, and halved mushrooms to the slow cooker.
In a small bowl, whisk together the beef broth, Dijon mustard, crushed peppercorns, and salt until the mustard dissolves.
Pour the broth mixture over the ingredients in the slow cooker and add the bay leaf.
Cover and cook on low for 8 hours until the beef is fork-tender.
Remove the bay leaf before serving.