Heat the oil in a large pot over medium heat. When the oil is hot, add the ground beef and ½ teaspoon of salt. Stir well and arrange it in a single layer. Cook for 2 minutes. Stir well, arrange in a single layer again, and cook for 2 more minutes. Then, continue cooking for 2-3 minutes while stirring and breaking up any large pieces until the beef is browned.
Depending on the fat content of the ground beef used it may be necessary to remove some of the oil rendered (see notes).
Next, place the pot over medium heat and add the diced onions and celery. Cook for 3-4 minutes, stirring frequently.
Add the garlic, tomato paste, oregano, and black pepper to the pot. Cook for 1-2 minutes, stirring frequently.
Then, add the diced tomatoes and beef broth, stir well while gently scraping any bits off the bottom of the pot. Raise the heat to high and bring the broth to a boil (it will take about 5-6 minutes). Then, lower heat to medium-low and cover the pot.
Cook the soup for 10 minutes, stir occasionally and keep the broth at a simmer. Raise or lower the heat as needed.
In the meantime, defrost the frozen vegetables by cooking them 1-2 minutes less than the lowest recommended time on the package. Drain them well and reserve.
Finally, add the alphabet pasta and the mixed vegetables to the pot and stir well.
Raise the heat and bring the broth to a boil (it will only take about 2 minutes). Then, lower the heat to medium-low, cover, and cook for 15 minutes until the pasta is tender.
Stir the soup occasionally to prevent any sticking on the bottom of the pot, especially when it is first added.
Taste the alphabet soup and add salt if needed. As a reference we added ½ teaspoon to ours.