Pizza Soup

Stove Top:

In a large pot over medium heat, cook the Italian sausage and diced onion until the sausage is browned and the onion is soft, about 5 minutes.
Add the mushrooms, green bell pepper, and garlic. Cook for another 3–4 minutes until the vegetables soften.
Stir in the oregano, salt, and pepper. Pour in the marinara sauce, diced tomatoes, and vegetable broth. Bring to a simmer and let cook for 15–20 minutes to blend flavors.
Stir in the black olives and pepperoni. Let simmer for another 5 minutes.
Ladle into bowls and top each serving with shredded mozzarella cheese.

Instant Pot:

Set the Instant Pot to Sauté mode. Add the ground Italian sausage and diced onion. Cook for 4–5 minutes, breaking up the sausage until browned.
Stir in mushrooms, green bell pepper, and garlic. Sauté for another 2–3 minutes until slightly softened. Add oregano, salt, and pepper. Stir to combine.
Pour in a splash of the vegetable broth and use a wooden spoon to scrape up any browned bits from the bottom of the pot.
Stir in marinara sauce, diced tomatoes, and the rest of the vegetable broth. Do not add the mozzarella, olives, or pepperoni yet.
Seal the lid and set to Manual/Pressure Cook on High for 5 minutes. Once done, do a quick release of the pressure.
Stir in mozzarella cheese, black olives, and halved pepperoni. Let sit for a few minutes to melt the cheese and warm the toppings.