Sauté onion, carrot & celery in olive oil with salt + pepper until softened.
Add pasta, toast 1–2 min until lightly golden, then toss in mushrooms.
Pour in chicken stock to cover by ~1 inch. Simmer like risotto, adding more stock/water as needed.
When pasta is just shy of al dente, stir in butter, spinach & splash of vinegar.
Serve topped with Parmigiano Reggiano.
