In a large pot heat olive oil over medium heat.
Add diced onion and sauté until softened and translucent, about 4-5 minutes.
Add garlic and cook, stirring constantly, until fragrant, about 1 minute.
Add salt, pepper, oregano and garlic powder.
Add tomato paste and stir until combined with the onions and spices.
Add the can of crushed tomatoes, beef broth, and milk to the pot. Stir to combine.
Add frozen meatballs and bring broth to a simmer.
Reduced heat and simmer, covered, for about 10 minutes, or until meatballs are heated through, stirring occasionally.
Add spinach leaves and stir to wilt them into the soup.
Add the Parmesan cheese, and half the Mozzarella cheese to the pot. Stir to combine and cook until cheese has completely melted into the broth.
Add crushed red pepper flakes.
Taste and season soup with additional salt and pepper if needed.
Remove pot from heat.
Ladle soup into oven safe bowls and place them on a cookie sheet.
Top each bowl of soup with slices of bread to cover the surface and top with a generous layer of Mozzarella cheese.
Place bowls in the oven and set temperature to broil. Broil until cheese is melted and lightly browned and bubbly, about 2-3 minutes.
Serve immediately.