In a large pot, heat the olive oil over medium heat. Add the diced onion and minced garlic, sautéing until the onion is translucent, about 3-4 minutes.
Add the ground beef to the pot, breaking it apart with a spoon. Cook until browned, about 5-7 minutes. Drain any excess fat.
Stir in the sliced carrots, chopped celery, and diced bell pepper. Cook for an additional 5 minutes, allowing the vegetables to soften.
Add the diced tomatoes (with juice), beef broth, oregano, basil, salt, and pepper. Bring the mixture to a boil, then reduce the heat and let it simmer for 20-25 minutes.
Stir in the frozen mixed vegetables and simmer for another 5-10 minutes until heated through. Adjust seasoning as needed.
Serve hot, and enjoy your hearty soup!