Heat the olive oil in a skillet over medium high heat.
Saute the chicken sausage until browned and cooked through. Remove and set aside.
Melt the butter in a pot over medium heat.
Stir in the onions and peppers, and saute until softened.
Mix in the garlic and Italian seasoning, and saute for 30 seconds longer.
Toss in the mushrooms, and saute for 2-3 minutes.
Pour in the diced tomatoes, tomato sauce and chicken broth, and simmer for 10 minutes uncovered.
Place the sausage in the pot, and simmer for another 5 minutes.
Stir in the parmesan until well combined.
Serve the soup topped with the mozzarella cheese if desired.
Notes
Sauté the veggies until they’re softened and aromatic before adding the liquids. This step enhances their flavor and gives the soup a richer, more well-rounded taste.
Browning the sausage first gives a deeper pizza flavor in the broth. Just make sure it's fully cooked.
Adding the mozzarella slowly helps it melt smoothly into the soup instead of clumping.
Control the broth consistency: If the soup is too thick, add a little extra chicken broth to achieve your desired consistency.