Heat the olive oil in a large wide pot or Dutch oven over medium heat. Add the sausage and cook 4–5 minutes, breaking it apart, until browned and cooked through. Remove and set aside, leaving the drippings in the pot.

Add the sliced onion and cook 3–4 minutes until softened. Add the sliced garlic and red pepper flakes and cook 1 minute until fragrant.

Add the cherry tomatoes and cook 2–3 minutes, pressing them gently with a spoon, until they begin to burst and release their juices.

Add the dry pasta and pour in the vegetable broth. Return the sausage to the pot and stir everything together, making sure the pasta is submerged as much as possible. Bring to a boil over medium-high heat.

Reduce heat to a vigorous simmer and cook 10–12 minutes, stirring every 2 minutes to prevent sticking and ensure even cooking. The pasta will absorb most of the broth and the sauce will become thick and creamy as the starch releases.

In the final 2 minutes, stir in the fresh spinach and let it wilt into the pasta.

When the pasta is tender and only a small amount of saucy liquid remains, remove from heat. Taste and adjust seasoning with salt and pepper.

Serve directly from the pot, topped generously with freshly grated Parmesan and fresh basil if desired.