Heat the Dutch oven over medium-high heat and brown the Italian sausage, breaking into crumbles, for 7 to 8 minutes until cooked.
Drain excess fat from cooked sausage, then add butter and minced garlic, stirring constantly for 1 minute until fragrant.
Pour chicken broth into the pot and bring the mixture to a rolling boil over high heat for about 5 minutes total.
Break lasagna noodles into bite-sized pieces and add to boiling broth, cooking for 10 to 12 minutes until tender.
Reduce the heat to low and stir in heavy cream, whole milk, Italian seasoning, salt, and black pepper until combined.
Gradually add mozzarella cheese and Parmesan cheese, stirring continuously until cheeses melt completely and sauce becomes smooth and creamy.
Fold in fresh spinach leaves and stir gently until wilted, which takes approximately 2 minutes of stirring time.
Ladle hot soup into serving bowls and garnish with extra Parmesan, fresh basil, or red pepper flakes as desired.