Heat olive oil in a large Dutch oven over medium-high heat. Add sausage, break up with a spoon, and cook until browned with crispy bits, about 6–8 minutes. Transfer to a plate, leaving drippings in the pot.

Reduce heat to medium. Add butter; once melted, toss in onion and cook until translucent, about 3–4 minutes. Add garlic and red pepper flakes; stir for 30–60 seconds until fragrant.

Pour in chicken broth and scrape up any browned bits from the bottom. Bring to a gentle boil.

Drop in broken lasagna noodles and stir to prevent clumping. Simmer for 8–10 minutes, stirring occasionally, until al dente.

Lower heat to medium-low. Stir in heavy cream, cream cheese, and nutmeg, patiently stirring until cream cheese melts and the broth turns velvety.

Return the sausage to the pot. Gradually whisk in parmesan by handfuls, stirring constantly to prevent clumping. Simmer for an additional 2–3 minutes.

Stir in spinach until wilted, about 1 minute. Season with salt and pepper, and add lemon juice to brighten.

Sprinkle mozzarella over the pot and let it melt, or ladle soup into bowls and top with mozzarella and extra parmesan.

Garnish with fresh basil or parsley before serving.