Season beef with salt and pepper. Brown in oil over medium-high heat in two batches, 5 to 6 minutes each.
Transfer to a 6-quart slow cooker; add onion, carrots, celery, mushrooms, and barley.
Whisk broth, tomato paste, Worcestershire, and thyme; pour into cooker. Add bay leaf.
Cover; cook on low for 7 to 8 hours until beef is tender and barley cooked through.
Discard bay leaf; stir in frozen peas.
Cover; cook on high for 8 to 10 minutes until peas are hot and bright green.
If barley has absorbed too much broth, thin with 1/2 cup hot water. Check salt.