Heat 1 tablespoon of olive oil in a large pot or Dutch oven over medium-high heat. Add the lean ground beef and break it up with a wooden spoon. Cook until the beef is fully browned, about 6-8 minutes. If there's excessive grease, carefully drain it off.

Reduce the heat to medium. Add the diced onion, chopped carrots, and chopped celery to the pot with the browned beef. Cook and stir until the onions are translucent and the carrots begin to soften, about 5-7 minutes.

Stir in the minced garlic, Italian seasoning, and dried thyme. Cook for about 60 seconds, until fragrant.

Pour in a splash of the beef broth and scrape up any browned bits from the bottom of the pot. Add the remaining beef broth, tomato sauce, fire-roasted tomatoes (undrained), bay leaf, and Worcestershire sauce. Stir well to combine.

Bring the mixture to a boil, then reduce the heat to low, cover, and simmer for 15-20 minutes, or until the diced potatoes are fork-tender.

Stir in the frozen mixed vegetables. Increase the heat slightly and cook uncovered for another 5 minutes, until the vegetables are heated through.

Remove the bay leaf. Taste and adjust seasoning with salt and pepper. Ladle into bowls, garnish with chopped parsley, and serve hot.