In the saucepan: bring water to a boil, add salt & cook the pasta until soft (about 15 min for mezzi rigatoni). Drain the pasta in the colander, keep the pasta warm & set aside.
In the meantime: Grate the Parmesan & set aside.
Trim & chop the scallions & set aside.
Trim & chop the mushrooms & set aside.
Melt the butter in the frying pan.
Fry the onions & the mushrooms - softly. Nothing shall get burnt.
Add the frozen peas - and some tablespoons of the pasta cooking water.
Add salt & pepper to taste.
After about 20 min add the finely grated Parmesan & mix.
Add the pasta & mix.
Serve at once. (You may serve grated Parmesan extra when serving!)