Heat olive oil in a large pot over medium-high heat. Add Italian sausage and cook, breaking into small pieces, until browned and crispy, about 6-8 minutes. Transfer to a plate.

In the same pot, add diced onion and cook for 3-4 minutes until softened. Add minced garlic and Italian seasoning, cooking for 30 seconds until fragrant.

Pour in chicken broth and diced tomatoes with juices. Scrape the bottom of pot to release browned bits. Bring to a boil.

Add uncooked orzo pasta, reduce heat to medium, and cook for 8-10 minutes, stirring occasionally, until pasta is tender.

Reduce heat to low and stir in heavy cream. Return cooked sausage to the pot with any accumulated juices.

Add chopped spinach and stir until wilted, about 2-3 minutes. Remove pot from heat.

Sprinkle mozzarella cheese over top and cover with lid for 2-3 minutes to melt.

Stir melted cheese into soup. Season with salt, black pepper, and red pepper flakes if desired. Serve immediately, garnished with fresh basil if available.