Fill a large pot with water and bring to a boil.
In a large saucepan, heat oil over medium-high heat. Fry rosemary until crisp, about 2 minutes. Remove from oil and place on paper towel to soak any excess oil. Set aside.
Add sausage to saucepan and cook until browned Scoop out sausage with a slotted spoon and set aside on a plate.
Add mushrooms to saucepan and cook until brown, about 5 minutes.
Add wine and cook until liquid is almost completely evaporated.
Meanwhile, cook the pasta until very al dente. *Reserve the cooking liquid.
With a slotted spoon, transfer cooked pasta to saucepan with mushrooms. Add spinach and 1 cup pasta cooking liquid.
Cook, tossing frequently, until spinach is wilted and sauce has thickened (about 4 minutes).
Add another cup of cooking liquid. Mix in grated cheddar and parmesan. Season with salt and pepper. Add a pinch of red pepper flakes if desired.
Add butter and toss to combine.
Gently mix in sausage.
Crumble fried rosemary over pasta and sprinkle with extra parmesan cheese. Serve immediately.