Heat 6-7 quart Dutch oven over medium-high heat. Pat beef cubes dry and sear for 5-6 minutes, turning every 1-2 minutes until deep brown crust forms on all sides. Remove and set aside.
In the same pot, add diced onion, carrots, and celery. Cook for 5 minutes, stirring occasionally, until vegetables soften and become slightly translucent. Add minced garlic and cook for 30 seconds until fragrant.
Return browned beef to pot along with rinsed barley, beef broth, tomato paste, thyme, bay leaves, and Worcestershire sauce. Stir to combine and dissolve tomato paste completely. Bring to a rolling boil.
Reduce heat to low, cover with lid, and simmer gently for 1 hour and 15 minutes, stirring every 15-20 minutes. Beef should become fork-tender and barley will triple in size.
If using mushrooms, add during the last 15 minutes of cooking time.
Remove and discard bay leaves. Taste and adjust seasoning with salt and pepper. Stir in balsamic vinegar if desired. Serve hot.
Notes
Use pearl barley for best texture and faster cooking. For gluten-free, substitute quinoa using same measurement.
Don’t overcrowd pot when browning beef or it will steam instead of sear. Brown in batches if needed.
Soup thickens as it sits. Add 1-2 cups extra broth or water when reheating leftovers.
Instant Pot method: Cook all ingredients on high pressure for 35 minutes with 10 minutes natural release.
Freezes well for up to 3 months in airtight containers.
A 6-7 quart Dutch oven works best for this recipe.